Anatomy & Physiology

Eleven words from a radiology report

A radiologist looks at the ankle X-ray of a sixteen-year-old who came off a skateboard and types one line: non-displaced fracture of the distal fibula at the lateral malleolus; medial malleolus intact. Every word is load-bearing. Distal says which end of the bone, lateral which side of the ankle, malleolus names the knob you can feel through the skin, and non-displaced says the fragments have not slid apart, which is the difference between a plaster boot and an operation.

A surgeon in Manila and a physiotherapist in Leeds read that sentence identically, because anatomy runs on a shared vocabulary: a fixed reference pose, a fixed set of direction words, and a small stock of Greek and Latin parts that recombine. Learn the system and you stop memorising terms one at a time.

One standard pose, so nobody has to ask

Every description assumes the body is in anatomical position: standing upright, feet slightly apart, eyes forward, arms at the sides with palms turned forward and thumbs pointing away from the body. People forget the palms, and it matters, because palms forward puts the radius on the thumb side and the ulna on the little-finger side, which is how the forearm bones are always named. Supine means lying face up, prone face down.

Key idea: left and right always mean the patient's left and right, so on an X-ray the patient's left is on your right. Getting this backwards is how the wrong limb gets treated, which is why surgical teams mark the site in ink before scrubbing in.

The direction words come in opposed pairs

There are only about six pairs to hold, and each one answers a different question.

Every one of these is relative: nothing is simply superior, it is superior to something else. The knee is distal to the hip and proximal to the ankle in the same breath.

Three planes, and why a scan is a stack of one of them

A plane is an imaginary flat cut, and there are three. The sagittal plane runs vertically and splits left from right; down the exact middle it is the midsagittal plane. The frontal or coronal plane also runs vertically but splits front from back. The transverse plane runs horizontally and splits superior from inferior. A CT scanner produces a stack of transverse slices a few millimetres apart and rebuilds the other two views from it, so when a radiologist says a tumour is best seen coronally, they mean which cut lays it out flat.

Cavities, and the slippery bags inside them

Organs sit in enclosed spaces. The dorsal cavity holds the brain in the cranial cavity and the spinal cord in the vertebral cavity. The larger ventral cavity is split by the diaphragm, the dome of muscle forming the floor of the chest: above it the thoracic cavity with the lungs and, between them in the mediastinum, the heart; below it the abdominopelvic cavity, digestive organs above and bladder and reproductive organs in the pelvis.

Each organ there is wrapped in a double-layered serous membrane, a sheet folded back on itself with slippery fluid between the layers: pleura around the lungs, pericardium around the heart, peritoneum around the abdominal organs. The layer on the organ is visceral, the layer lining the wall is parietal, and that fluid is why your lungs slide against your ribs without wearing out. Clinicians also split the abdomen into four quadrants by lines crossing at the navel; right lower quadrant pain is the classic clue to appendicitis.

Reading a word you have never seen

Medical terms are built like Lego: a prefix that modifies, a root naming the structure, a suffix saying what is happening to it. Learn thirty parts and you can decode hundreds of words.

Try one cold. Hyperkalaemia: hyper- (above normal) plus kali- (potassium, from Latin kalium, which is why its symbol is K) plus -aemia (in the blood). Too much potassium in the blood, defined by someone who has never seen the word.

Back to the ankle

Now reread the report. The fibula is the slim outer bone of the lower leg, and distal means the end further from the knee, so the break is at the ankle. The lateral malleolus is the bump on the outer side, the fibula's bottom end; the medial malleolus is the inner bump and belongs to the tibia. A break at the lower end of the outer leg bone, still in position, inner side unharmed. Eleven words, one unambiguous picture.

The body defends a range, not a number

Anatomy tells you where things are. Physiology asks what they do, and almost every answer is a version of one trick: hold an internal variable steady while the outside world does whatever it likes. That is homeostasis, from the Greek homoios (similar) and stasis (standing still). The word misleads slightly, because nothing stands still. Values oscillate inside a range, and the body spends energy pushing them back when they drift.

Take temperature. The figure everyone quotes, 37 degrees Celsius or 98.6 degrees Fahrenheit, comes from Carl Wunderlich's measurements in nineteenth-century Leipzig. Modern sources put normal core temperature at about 37 plus or minus 0.5 degrees Celsius. And the average has been falling: Protsiv and colleagues pooled measurements from three American datasets spanning 157 years, from Union Army veterans to Stanford clinic patients in 2007 to 2017, and found mean body temperature dropping about 0.03 degrees Celsius per birth decade, so men born in the early 1800s ran roughly 0.59 degrees warmer than men measured today. Even a textbook constant has a history.

The loop, in four parts

Every homeostatic control has the same architecture, and once you can name the parts you can find them in any system in this course.

1. A stimulus: the variable drifts out of range.
2. A receptor detects the drift. For temperature these are thermoreceptors in the skin and neurons in the preoptic area of the hypothalamus that sample the blood passing through.
3. A control centre compares that value with the set point. For temperature it is the hypothalamus itself.
4. An effector carries out a correction that opposes the original change, which is what makes it negative feedback.

Thermoregulation, worked both ways

You run on a warm day and your core temperature climbs. The hypothalamus registers warmer blood and does two things. It reduces sympathetic signalling to the small arteries of the skin, letting them widen: vasodilation. Warm blood reaches the surface and heat leaves, which is why a running face goes red. It also drives the sweat glands. Sweat itself does not cool you; the evaporation does, because turning liquid water into vapour takes energy out of the skin. That is why a humid day is worse: the sweat pools instead of evaporating and the mechanism stalls.

Now the reverse, at a bus stop in January. Cooler blood reaches the hypothalamus, sympathetic signalling to skin arteries increases, and they narrow: vasoconstriction. Blood is held in the core, less heat is lost, and your fingers go pale. If that is not enough, the hypothalamus drives skeletal muscle into rapid involuntary contraction, which is shivering, and muscle is inefficient enough that most of that energy leaves as heat.

The upshot: in every negative feedback loop the response opposes the disturbance. Hot leads to heat loss, cold to heat conservation and heat production.

Fever is not a broken thermostat

When an infection is present, immune cells release pyrogens that act on the hypothalamus and raise the set point, to 39 degrees Celsius say. The thermostat works perfectly; it is defending a higher number. That explains the strangest part of a fever: at 39 degrees you feel freezing and shiver under a blanket, because relative to the new set point you are too cold. When the infection clears the set point drops, you are above it, and you sweat heavily. That is the fever breaking. Hyperthermia is different, with no set-point change: heat stroke is the loop overwhelmed rather than reset, which is why cooling treats it and paracetamol, which lowers a raised set point, does not.

When feedback pushes instead of pulls

Positive feedback amplifies the original change instead of reversing it, and it always needs an outside event to stop. In childbirth the baby's head stretches the cervix, stretch receptors trigger oxytocin release, the uterus contracts harder, and the head presses harder still, escalating until delivery removes the stimulus. Clotting is the same: activated platelets recruit more platelets until the breach is sealed.

Common misconceptions

• Normal body temperature is exactly 98.6 degrees Fahrenheit. It is a nineteenth-century average. Yours swings about half a degree Celsius over a day, lowest in the early morning, and the population mean has fallen since Wunderlich.
• Sweating cools you. Evaporation cools you. Sweat that drips off your chin has removed almost no heat, which is the problem with humidity.
• A fever means temperature control has failed. A fever is that control defending a higher target. Failure looks like heat stroke or hypothermia instead.

Putting it together

Anatomy describes position from one agreed pose, and left and right belong to the body being described. Six pairs of relative terms locate anything and three planes cut it, the transverse being the one a CT scanner images directly. Organs sit in the dorsal and ventral cavities, separated by the diaphragm and wrapped in serous membranes whose fluid stops friction. Unfamiliar terms are decoded rather than memorised, by splitting off prefix, root and suffix. Physiology then runs on negative feedback: a receptor detects drift, a control centre compares it with a set point, an effector opposes the change. Thermoregulation is the model case, and fever is that loop defending a deliberately raised target. Worth holding on to: almost every mechanism in the next sixteen lessons is a version of this loop.

Sources

1. OpenStax. (2022). 1.6 Anatomical terminology. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 1.5 Homeostasis. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Osilla, E. V., Marsidi, J. L., Shumway, K. R., and Sharma, S. (2023). Physiology, temperature regulation. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. Protsiv, M., Ley, C., Lankester, J., Hastie, T., and Parsonnet, J. (2020). Decreasing human body temperature in the United States since the Industrial Revolution. eLife, 9, e49555. eLife

Five micrometres of skin on a glass slide

A pathology lab takes a piece of skin, sets it in wax, and shaves off a slice about five micrometres thick, thinner than a red blood cell is wide. The slice goes into two dyes. Haematoxylin binds to acidic material, above all the DNA in nuclei, and turns them blue-purple. Eosin binds to proteins in the cytoplasm and to collagen, and turns them pink. Under the microscope you now see a pink and purple landscape, and it is not decorative: every structure you will meet in this course was first identified this way.

What the slide shows is layers. At the top, flat dead cells packed with keratin. Below them, living cells stacked several deep with purple nuclei. Below those, a looser pink tangle with only scattered nuclei, threaded by circles that turn out to be blood vessels cut across. Two completely different construction methods, sitting on top of each other. Learning to tell them apart is what this lesson is for.

What every human cell has, and what only some have

Your body runs on roughly thirty trillion cells of about two hundred kinds, and they share a standard kit. The plasma membrane is a double sheet of phospholipids that decides what enters and leaves. The nucleus holds the DNA and contains a dense spot, the nucleolus, where ribosomes are assembled. Mitochondria burn fuel with oxygen to make ATP, and they carry their own small loop of DNA, inherited only from your mother. Ribosomes build proteins, either free in the cytoplasm or studded on the rough endoplasmic reticulum; the smooth ER makes lipids and stores calcium, which will matter enormously when we get to muscle. The Golgi apparatus sorts and packages finished proteins, lysosomes digest worn-out parts, and the cytoskeleton holds the shape.

Then cells specialise, sometimes drastically. A mature red blood cell ejects its nucleus and its mitochondria, which is why it cannot repair itself and lasts only about 120 days, and why it does not consume the oxygen it is carrying. A motor neuron running from your spinal cord to your big toe is a single cell around a metre long. A skeletal muscle fibre has dozens of nuclei because it formed by many cells fusing. A white blood cell can crawl out through a capillary wall. Structure follows the job, every time.

Four tissues, and how to tell them apart

A tissue is a group of cells with a common origin working together. There are exactly four primary types, and almost every organ is a particular arrangement of all four. The fastest way to sort them is to ask one question of the slide: how much of what you are looking at is cells, and how much is the stuff between the cells?

Epithelium: sheets with a top, a bottom and no blood supply

Epithelium always has polarity: an apical surface facing a space or the outside world, and a basal surface glued to a thin sheet of protein called the basement membrane. It has no blood vessels of its own, so every nutrient diffuses up from the connective tissue underneath, which caps how thick an epithelium can get. It also divides constantly to replace losses, and constant division is why most adult cancers begin in epithelium; a cancer of epithelial origin is called a carcinoma.

Naming an epithelium takes two words: how many layers, then what shape the surface cells are.

• Simple squamous, one layer of flat tiles, so thin that gases cross it. This is the wall of an alveolus in the lung and the lining of every capillary.
• Simple columnar, one layer of tall cells, often with microvilli making a fuzzy brush border. This lines your small intestine, where surface area is the whole point.
• Pseudostratified ciliated columnar, one layer whose nuclei sit at different heights so it looks stacked, carrying beating cilia. This lines the trachea and sweeps mucus upward.
• Stratified squamous, many layers with flat cells on top, built to survive abrasion. Keratinised in the epidermis, non-keratinised and kept moist in the mouth and oesophagus.

Why this matters: you can predict the epithelium from the job. Somewhere that must let gases through gets one flat layer. Somewhere that gets scraped gets many layers. Somewhere that absorbs gets tall cells with microvilli.

Connective tissue: the cells are the minority

Here the rule reverses. Connective tissue is mostly matrix, which the cells secrete and then sit in: a ground substance of water and sugars, plus fibres. Collagen fibres resist pulling and are the most abundant protein in your body. Elastic fibres snap back after stretch. Reticular fibres form fine supporting nets in organs like the spleen.

Change the matrix and you change the tissue completely. Loose areolar tissue has a soft watery matrix and packs the gaps between organs. Adipose tissue is cells so full of stored fat that the nucleus is squashed against the wall. A tendon is dense regular tissue with collagen in parallel bundles, all aligned with the pull. The dermis is dense irregular tissue with collagen woven in every direction, because skin gets pulled from every direction. Cartilage has a rubbery matrix and, crucially, no blood vessels, which is why a torn knee cartilage heals so badly. Bone has the same protein framework with calcium phosphate crystals packed into it. And blood is a connective tissue whose matrix is liquid: plasma, with cells suspended in it.

Muscle and nerve: the two that are electrically excitable

Muscle comes in three kinds and you can tell them apart on sight. Skeletal muscle fibres are long cylinders with obvious cross-stripes and many nuclei pushed out to the edge, and they are under voluntary control. Cardiac muscle cells are shorter, branched, striated, with one or two central nuclei, joined end to end by dark lines called intercalated discs, and they are involuntary. Smooth muscle cells are spindle-shaped with a single central nucleus and no stripes at all; they line the gut, the airways and the blood vessels, and are involuntary.

Nervous tissue is two populations. Neurons generate and transmit electrical signals and have a cell body with branching dendrites and one long axon. Neuroglia, the smaller cells crowded around them, insulate axons, mop up chemicals and defend against infection. On a slide, the neurons are the big ones and the glia are the many small dark nuclei in between.

Reading three slides back

Now use the table. Slide one: almost no cells, dense pink fibres running in perfectly parallel bundles, with flattened nuclei squeezed between them. Connective tissue, dense and regular, aligned with a single line of pull: a tendon. Slide two: a single layer of tall cells with a fuzzy free border, folded into deep valleys, sitting on a thin line with blood vessels beneath. Simple columnar epithelium with microvilli: small intestine. Slide three: long striped cells lying parallel, with nuclei flattened against the outer edge of each. Skeletal muscle, and the position of the nuclei is what tells you it is not cardiac.

Where the four come from

Three weeks after fertilisation the embryo has folded into three sheets. Ectoderm, the outer sheet, becomes the epidermis and the whole nervous system, which is why your skin and your brain are cousins. Mesoderm, the middle sheet, becomes connective tissue, bone, cartilage, blood and muscle. Endoderm, the inner sheet, becomes the linings of the digestive and respiratory tracts. Epithelium is the exception that arises from all three.

Common misconceptions

• Blood is not a tissue. Blood is a connective tissue. Its cells were made in bone marrow and sit in a matrix, and the only unusual thing is that the matrix is a liquid.
• Cartilage heals like bone. Bone has a rich blood supply and repairs a fracture in weeks. Cartilage has none, so its cells are fed by slow diffusion and a torn meniscus may never repair itself.
• Epithelium means skin. Skin is one epithelium. So is the lining of your gut, your airways, your bladder and every gland in your body.
• All your cells are much the same size. A red blood cell is about 8 micrometres across and a single motor neuron can stretch a metre.

The short version

Cells share one kit of parts, then specialise so hard that a red blood cell throws away its nucleus and a neuron grows a metre long. Four tissues build everything, and the question that sorts them is how much matrix lies between the cells. Epithelium is nearly all cells, packed into sheets with a free apical surface, a basement membrane below and no blood supply of its own, classified by layers and by cell shape. Connective tissue is mostly matrix, and changing the matrix turns the same basic design into tendon, fat, cartilage, bone or blood. Muscle is long contractile cells in three recognisable varieties, and nervous tissue is neurons among far more numerous glia. The core of it: every organ in the next fifteen lessons is these four tissues arranged differently, so if you can name a tissue from its appearance, you can usually work out what the organ does.

Sources

1. OpenStax. (2022). 4.1 Types of tissues. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 4.2 Epithelial tissue. In Anatomy and Physiology 2e. Rice University. openstax.org
3. OpenStax. (2022). 4.3 Connective tissue supports and protects. In Anatomy and Physiology 2e. Rice University. openstax.org
4. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. (2002). Molecular Biology of the Cell (4th ed.). Garland Science. NCBI Bookshelf

Fifteen to thirty layers of dead cells

Drag a fingernail slowly across the back of your other hand and the fine pale dust that lifts off was you. The outermost sheet of your epidermis, the stratum corneum, is usually 15 to 30 layers of cells that are no longer alive: flattened bags of the fibrous protein keratin, mortared together with lipid, shed constantly and replaced from below on a cycle of about four weeks. Over most of your body that dead sheet is thinner than a page of a book, and it is the whole barrier between your bloodstream and every microbe in the room.

Which is why a large burn is an emergency in a way that a large bruise is not. Remove that barrier over a third of the body and water, heat and infection control fail together, within hours, before anything else has gone wrong. To see how that happens you have to build the skin first, from the bottom.

The epidermis, bottom to top

The epidermis is keratinised stratified squamous epithelium: many layers of cells, flattened as they rise, the top ones filled with keratin and dead. Like every epithelium it has no blood vessels of its own. Everything it needs diffuses up from the dermis underneath, and that diffusion distance is what caps its thickness. New cells are made only at the bottom, and they are pushed upward, dying on the way.

What matters here: a structure with no blood supply that rebuilds itself every four weeks can only be fed from below, which is why damage reaching the dermis is a different problem from damage that stops short of it.

Where skin colour comes from

Scattered along the stratum basale are melanocytes, whose branching arms reach up between the keratinocytes and hand over packets of melanin called melanosomes. The keratinocyte parks that pigment on the sunward side of its own nucleus, a parasol over the DNA.

Human populations differ very little in how many melanocytes they have. What differs is how much melanin each one makes, which type, and how the melanosomes are packaged as the cell rises. Melanin absorbs ultraviolet photons before they reach DNA, so darker skin carries more built-in UV protection and correspondingly less capacity to make vitamin D from weak winter sunlight. A tan is not a sign of health; it is pigment production stepped up because DNA damage has already happened.

The dermis is where the skin actually lives

Under the basement membrane is the dermis, which makes up most of the thickness of skin and comes in two layers. The papillary layer is loose connective tissue thrown up into ridges, the dermal papillae, which lock the epidermis down and carry the capillary loops that feed it; on your fingertips those ridges are your fingerprints, unchanged after the epidermis above them has been replaced fifty times. The reticular layer below is dense irregular connective tissue, thick collagen bundles running every which way with elastin woven through. Collagen resists the pull, elastin snaps the skin back, and stretch marks are tears in that collagen seen through an intact epidermis.

The dermis holds the working parts: vessels, nerve endings, follicles, sweat glands, sebaceous glands. Below it, not counted as skin proper, is the hypodermis, mostly fat, which insulates, cushions and anchors skin to the fascia over muscle and bone.

Hair, nails and two kinds of sweat gland

A hair is a column of dead keratinised cells pushed out of a follicle by a living bulb, lengthening about 0.3 millimetres a day while it grows. Each follicle runs its own cycle, two to seven years growing then two to four months resting, and because the cycles are out of step you shed roughly fifty hairs a day without noticing. A tiny smooth muscle on each follicle, the arrector pili, tilts it upright: that is a goose bump, a heat-trapping reflex that works on a furred animal and does almost nothing on us.

Two gland types share the dermis and are constantly confused. Eccrine glands open straight onto the surface, are densest on palms, soles and forehead, and secrete a watery hypotonic sweat carrying salt and antimicrobial peptides; these are the cooling glands. Apocrine glands sit deeper, empty into hair follicles in armpit and groin, switch on at puberty, and release a thicker secretion that is odourless until skin bacteria break it down, so body odour is bacterial metabolism rather than sweat. Sebaceous glands also empty into follicles, releasing sebum whose fatty acids limit water loss and inhibit some bacteria; they wake at puberty too, which is why acne has the age distribution it has.

Four jobs, and what each one depends on

Barrier. Keratin and lipid stop water leaving and most microbes entering, sebum adds fatty acids, and Langerhans cells sample whatever gets through.

Temperature. At rest about 500 millilitres of water a day leaves through the skin unnoticed, which is why it is called insensible perspiration; someone working hard in heat can produce 0.7 to 1.5 litres of sweat an hour. Add dermal vessels widening or narrowing on orders from the hypothalamus and the skin is the body's main radiator.

Sensation. Merkel cells in the stratum basale and tactile corpuscles in the dermal papillae report light touch; lamellated corpuscles, onion-shaped and deep in the dermis, report vibration and pressure; bare nerve endings report pain and temperature. Their density varies enormously, which the activity below lets you measure on yourself.

Vitamin D. Ultraviolet B light converts a cholesterol derivative in the skin into cholecalciferol, vitamin D₃. The liver turns that into calcidiol and the kidney turns calcidiol into calcitriol, the active form that drives calcium absorption from the gut. Remember: the skin starts vitamin D and cannot finish it, so this one function chains skin to liver to kidney to bone, and we meet it again in the lesson on the skeleton.

Burns, classified by how deep they go

Now take the structure apart. A burn is graded by depth, not by what caused it, because depth decides everything that follows.

The pain pattern is the counter-intuitive one, and it is diagnostic. A partial thickness burn hurts appallingly because the nerve endings are damaged but alive. A full thickness burn over its centre may hurt hardly at all, because the endings have been destroyed along with everything else in that dermis. Painlessness is a bad sign, not a good one.

Healing follows the same logic. Partial thickness skin regrows from the epithelium lining the deep parts of hair follicles and sweat glands, reservoirs of living basal cells buried in the dermis. Destroy the dermis and those reservoirs go with it, so a full thickness wound can only close by contracting inward and filling with scar, which is why surgeons cover it with a skin graft from an unburned site.

Working one burn end to end

Extent is estimated with the Wallace rule of nines, which divides the adult body surface into blocks of nine percent: the head 9, each arm 9, the front of the trunk 18, the back of the trunk 18, each leg 18, and the perineum 1. Total 100. Superficial burns are not counted in the figure used for fluid calculations.

Take an adult of 70 kilograms with partial and full thickness burns covering the whole of one arm, the front of the trunk, and the front of one leg. Add the blocks: 9 for the arm, 18 for the front of the trunk, 9 for the front of the leg, since one leg is 18 in total and the front is half of it. That is 36 percent of total body surface area.

The Parkland formula then sets the first day of fluid replacement: 4 millilitres of Ringer's lactate for each kilogram of body weight for each percent burned, given over 24 hours, with half of it in the first 8 hours. Multiply it out: 4 times 70 times 36 gives 10,080 millilitres, just over ten litres, of which 5,040 millilitres must go in during the first eight hours. Two details decide whether that number is any use. The clock starts at the moment of the burn, not at the moment the patient reaches hospital, so an hour lost in transport compresses the remaining schedule. And the formula is a starting estimate that is then adjusted against how much urine the patient makes, because urine output is the cheapest bedside signal that the kidneys are still being perfused.

Why a large burn kills

Heat damages capillary walls, and damaged capillaries leak plasma into the tissues. Over a large area that leak is fast enough to empty the circulation: blood volume falls, blood pressure falls, organs stop receiving flow. That is hypovolaemic shock, and it is what the Parkland fluids are racing. The patient also cannot hold heat, because the radiator has lost its cover, and cannot keep bacteria out, because the barrier has gone; burn wound infection remains a leading cause of later death.

The American Burn Association treats several categories as needing a specialist burn centre: partial thickness burns over 10 percent of body surface, any full thickness burn, burns of the face, hands, feet, genitalia or major joints, and chemical, electrical and inhalation injuries. Hands and joints are on that list for a reason that is pure anatomy. Scar tissue contracts as it matures, and a contracting scar across the front of an elbow will pull the joint shut permanently unless it is grafted and splinted.

Common misconceptions

• A burn that does not hurt is a mild burn. The reverse. Loss of sensation over the burn means the dermal nerve endings have been destroyed, which puts it at full thickness.
• Blisters should be popped to let a burn breathe. The blister roof is sterile cover over living dermis, and removing it exposes the basal cells the wound has to heal from.
• Dark skin does not need sun protection. More melanin does mean more built-in UV absorption, but skin cancer still occurs in darkly pigmented skin and is more often found late.

Where this leaves us

Skin is an epithelium with no blood supply of its own, sitting on a connective tissue that has everything: vessels, nerves, glands and follicles. The epidermis makes cells at the stratum basale and sheds them from the stratum corneum about a month later, waterproofed in the granular layer on the way. Melanocytes hand pigment to keratinocytes, and population differences lie in melanin output rather than melanocyte count. The dermis feeds the epidermis, houses the sensors, and holds the reservoirs a partial thickness burn heals from. Burn depth therefore predicts pain, healing and treatment, extent comes from the rule of nines, and fluid comes from 4 millilitres per kilogram per percent, half in eight hours. Bottom line: all four of skin's jobs fail together when the dermis goes, and that simultaneity is what makes a big burn a whole-body emergency.

Sources

1. OpenStax. (2022). 5.1 Layers of the skin. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 5.3 Functions of the integumentary system. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Schaefer, T. J., and Szymanski, K. D. (2023). Burn evaluation and management. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. MedlinePlus. (2024). Burns. National Library of Medicine. MedlinePlus

The pattern inside a sawn femur

Saw a thigh bone lengthwise and the spongy interior of its upper end is not a random foam. The bony struts fan out from the neck of the femur in two crossing arcs, one arching up from the inner shaft toward the top of the head, one sweeping out toward the greater trochanter. Julius Wolff published the observation in 1892: the struts line up along the directions in which the bone is actually loaded. Bone adds material where force is high and removes it where force is low, which is why the pattern is an engineering drawing of a lifetime of standing up.

That is the fact to hold before anything else in this lesson. A skeleton is not scaffolding installed once. It is a tissue with a blood supply, a nerve supply, cells that build and cells that demolish, and an accounting system for calcium that the rest of the body draws on daily.

Two kinds of bone tissue, made of the same material

Every bone is built from two arrangements of one matrix. That matrix is collagen fibres stiffened by crystals of hydroxyapatite, a calcium phosphate mineral. The crystals supply hardness, the collagen supplies the give. Take the mineral out with acid and the bone becomes rubbery enough to tie in a knot; burn the collagen out and it becomes so brittle it crumbles under a thumb. Bone works because it is both at once.

Picture one osteon as the growth rings of a tree trunk seen end on, with a tunnel down the middle. Trapped between the rings, in small cavities called lacunae, sit the bone cells, connected to each other and to the central canal by hair-thin channels called canaliculi. Those channels matter: bone cells are walled into solid mineral, and canaliculi are the only way food and signals move between them.

The cavity in the shaft of a long bone holds marrow. Red marrow makes blood cells; yellow marrow is mostly fat. In a newborn nearly all marrow is red. In an adult, red marrow has retreated to the sternum, ribs, vertebrae, pelvis and the very ends of the femur and humerus, which is why a bone marrow sample is taken from the back of the pelvis and not from an arm.

The four cells that argue over every gram

Bone is remodelled continuously by four cell types whose names decode cleanly once you split them. The root oste- is bone. -blast is a builder, -cyte a resident cell, -clast a breaker.

• Osteogenic cells are the stem cells; they divide and become osteoblasts.
• Osteoblasts secrete collagen matrix and start its mineralisation. When one walls itself into the matrix it has just made, it becomes an osteocyte.
• Osteocytes are the mature residents in the lacunae. They sense mechanical load and signal for matrix to be added or removed, which is the cellular machinery behind Wolff's observation.
• Osteoclasts are large, multi-nucleated cells descended from the white cell line. They seal onto bone, pump out acid and enzymes, and dissolve matrix, releasing calcium into the blood.

Why this matters: bone mass at any moment is the running balance between osteoblast deposition and osteoclast resorption. Every disease and every treatment later in this lesson works by shifting that balance one way or the other.

206 bones, in two divisions

The adult skeleton has 206 bones. The axial skeleton is the 80 bones along the midline: skull, vertebral column, ribs and sternum. The appendicular skeleton is the other 126: the limbs and the two girdles that hang them from the axis.

Two notes that catch people out. The radius is on the thumb side and the ulna on the little-finger side, which only makes sense in anatomical position with the palm forward. And the fibula carries little body weight; it is the tibia that transmits load from knee to ankle, which is why a broken fibula is often walked on and a broken tibia is not.

Where bones meet

Joints are classified twice over, once by what holds them together and once by how much they move. The two classifications track each other closely: the more tissue between the bones, the less movement.

The synovial joints are the ones that do the work, and they come in six shapes. A hinge moves in one plane: the elbow, the knee, the finger joints. A pivot rotates around a long axis: the atlas turning on the axis so you can shake your head no. A ball and socket moves in every direction: shoulder and hip. A saddle joint, where two curved surfaces sit across each other, gives the thumb its opposition. A condyloid joint, oval in a shallow cup, moves in two planes: the wrist. A plane joint lets flat surfaces glide: between the small bones of the foot.

Sketch a synovial joint in your head. Two bone ends, each capped with glassy articular cartilage a few millimetres thick. Around them a fibrous capsule, lined on its inside by a synovial membrane that secretes a slippery fluid into the gap. Outside the capsule, ligaments, which run bone to bone, are not to be confused with tendons, which run muscle to bone. In the knee, add two crescents of tough fibrocartilage, the menisci, that deepen the flat top of the tibia, plus small fluid sacs called bursae where tendons rub. Articular cartilage has no blood supply and almost no capacity to repair, which is why cartilage injuries persist and why osteoarthritis, the wearing through of that cap, is the commonest joint disease of later life.

Healing a broken fibula, stage by stage

A fracture is named by the line it takes and whether it reaches air. A transverse break runs straight across, an oblique one at an angle, a spiral one twists around the shaft, a comminuted one shatters into three or more pieces, an impacted one drives one fragment into the other, and a greenstick break bends and splits only one side, which happens in children because their bone still has more collagen and less mineral. A closed fracture leaves the skin intact; an open or compound fracture breaks through it, and because that connects the bone to outside bacteria, it is a surgical emergency rather than a plaster case.

Repair then proceeds through four stages, each visible on sequential X-rays.

1. Fracture haematoma. Vessels torn inside the bone bleed, and about six to eight hours after the injury the blood clots into a mass sealing the gap. Bone cells near the break lose their blood supply and die.
2. Callus formation. Within about 48 hours, cells inside the bone build an internal callus while cells under the periosteum build an external one. Both are cartilage and fibrous tissue at first, splinting the fragments but not yet strong.
3. Cartilage replaced by bone. Over the next several weeks osteoclasts clear dead bone while osteoblasts convert the callus to trabecular bone. The lump is visible under the skin and on film.
4. Remodelling. Internal and external calli unite, compact bone replaces spongy bone at the margins, and the surplus is slowly removed along the lines of loading. In a child the shaft can end up with no visible trace.

The point: a cast does not heal a fracture. It holds the fragments still enough for those four stages to run, which is also why a break that will not stay aligned gets a plate, a screw or an intramedullary nail instead.

Bone as the calcium bank

Blood calcium is held near 10 milligrams per decilitre, and the margin for error is small: too little and muscles cramp, nerves misfire and blood clots poorly; too much and the nervous system slows into lethargy, constipation and confusion. Since almost all the body's calcium is in bone, bone is the reserve the body raids.

Two hormones run the account. Parathyroid hormone, released when blood calcium falls, drives osteoclasts to resorb bone, tells the kidney to reabsorb calcium rather than lose it in urine, and promotes activation of vitamin D so the gut absorbs more. Calcitonin, from the thyroid, pushes the other way by inhibiting osteoclasts. That is the vitamin D chain from the skin lesson arriving at its destination: without the final kidney step, dietary calcium is poorly absorbed however much of it you eat.

Osteoporosis: losing the argument slowly

Osteoporosis is what happens when resorption outruns deposition for years. Bone mineral density falls and the trabecular lattice loses struts, so the remaining bone is not just lighter but worse arranged. Fractures then occur under loads that would not have broken anything at twenty, most often at the hip, the vertebrae and the wrist. Estimates put more than 200 million people worldwide with the condition, and roughly one woman in three and one man in five over the age of 50 will have an osteoporotic fracture.

Diagnosis uses dual-energy X-ray absorptiometry, a low-dose scan of hip and spine. The result is reported as a T-score: the number of standard deviations the patient's bone density sits above or below that of a healthy young adult. Between negative 1 and negative 2.5 is osteopenia; below negative 2.5 is osteoporosis. The hip measurement predicts fracture better than the spine, because arthritic calcification in an older spine can push the spinal reading falsely high. Screening is generally recommended from age 65 in women and 70 in men, earlier where risk factors stack up.

Treatment attacks the same balance. Adequate dietary calcium and vitamin D supply the raw material; weight-bearing exercise supplies the loading signal that tells osteocytes to keep the matrix; stopping smoking and limiting alcohol removes two things that suppress bone formation. Drug treatment starts with bisphosphonates such as alendronate, risedronate or zoledronic acid, which bind into the mineral and poison osteoclasts that try to resorb it, or with denosumab, an antibody that blocks the signal osteoclasts need to mature. All of them work by slowing the demolition crew rather than by hiring more builders.

Common misconceptions

• Bone is dead, dry material. It has a blood supply, a nerve supply and four living cell types, and a healthy adult skeleton is rebuilt continuously; roughly a tenth of it is replaced in a year.
• Ligaments and tendons are the same thing. Ligaments join bone to bone across a joint. Tendons join muscle to bone. Both are dense regular connective tissue, which is why they look similar on a slide.
• Drinking milk in adulthood will rebuild lost bone. Calcium is necessary but not sufficient. Without loading and, in osteoporosis, without drug treatment, extra dietary calcium does not restore a depleted lattice.

Pulling it together

Bone matrix is collagen for flex and hydroxyapatite for hardness, arranged as osteons in compact bone and as stress-aligned trabeculae in spongy bone. Four cells run it: osteogenic stem cells, building osteoblasts, resident osteocytes that sense load, and demolishing osteoclasts. The 206 bones divide into an axial 80 and an appendicular 126, and joints are fibrous, cartilaginous or synovial, with the six synovial shapes accounting for nearly all free movement. A fracture heals through haematoma, callus, conversion to bone and remodelling, and immobilisation exists only to let that sequence run. Bone doubles as the calcium reserve, drawn on by parathyroid hormone and defended by calcitonin. In short: the skeleton is a balance sheet, and osteoporosis is what a long run of small deficits looks like on a DXA scan.

Sources

1. OpenStax. (2022). 6.3 Bone structure. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 6.5 Fractures: bone repair. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Porter, J. L., and Varacallo, M. A. (2023). Osteoporosis. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. National Institute of Arthritis and Musculoskeletal and Skin Diseases. (2023). Osteoporosis. National Institutes of Health. NIAMS

Why a dead muscle goes stiff

Rigor mortis can begin as early as four hours after death, peaks at around thirteen hours, and releases some two to two and a half days after that peak. The body becomes rigid, and the rigidity passes only when enzymes start to digest the machinery holding it. That is a strange result, because muscle contraction is famously expensive. It runs on ATP. A corpse makes none. Why should the muscle of a body with no energy supply be locked tight rather than floppy?

Hold that question. By the end of this lesson you will be able to answer it in one sentence, and the answer is the cleanest way into how a muscle actually works.

From whole muscle down to sarcomere

Your biceps brachii is a bundle of bundles. Wrapped in connective tissue, it divides into fascicles, the visible grain you see in a cut steak. Each fascicle is a bundle of muscle fibres, and here is the first surprise: one fibre is one cell, up to several centimetres long, formed in development by many cells fusing, so it holds hundreds of nuclei pushed against its membrane. That membrane has its own name, the sarcolemma, from Greek sarx, flesh.

Inside the fibre, filling almost all of it, are myofibrils: long cylinders of protein running the length of the cell. A myofibril is a chain of identical units joined end to end, and that unit is the sarcomere, roughly two micrometres long at rest. Everything a muscle does is one sarcomere getting shorter, multiplied by about ten thousand of them along a fibre and by hundreds of thousands of fibres.

Two more structures matter, and both are about calcium. The sarcoplasmic reticulum is smooth endoplasmic reticulum wrapped around every myofibril, and it is a calcium store. Transverse tubules, or T tubules, are inward folds of the sarcolemma that dive into the fibre and run alongside that store. Their point is speed: an electrical signal on the surface reaches the deepest myofibril at almost the same instant as the nearest one.

Reading the stripes

Skeletal muscle looks striped because the filaments inside a sarcomere are lined up. Thin filaments, made of actin with two regulatory proteins wound along them, anchor at each end of the sarcomere on a protein plate called the Z disc. Thick filaments, made of myosin, float in the middle, overlapping the thin ones without touching the Z discs.

Draw it across the page. Z disc at the left, thin filaments reaching right; thick filaments in the centre; thin filaments coming back in from the Z disc on the right. The pale I band is where thin filaments sit alone. The dark A band spans the full length of the thick filaments. Inside the A band, the H zone is the strip where thick filaments have no thin filaments beside them.

Now the decisive observation, made in 1954 by two independent teams who gave us the sliding filament account. When a muscle shortens, the I band narrows and the H zone narrows, but the A band keeps exactly the same width. If the filaments themselves were contracting, the A band would shrink too. It does not. So the filaments do not shorten; they slide past each other, and the Z discs are dragged closer together. Key idea: a muscle shortens by overlap, not by compression.

The synapse that hardly ever fails

A muscle fibre does nothing until a motor neuron tells it to. The meeting point is the neuromuscular junction, and the sequence runs like this.

1. An action potential arrives at the axon terminal and opens calcium channels there.
2. Calcium entry makes vesicles fuse with the terminal membrane and empty acetylcholine into the synaptic cleft, a gap of a few tens of nanometres.
3. Acetylcholine crosses and binds receptors on the motor end plate, the specialised patch of sarcolemma opposite the terminal. Those receptors are channels; they open, sodium floods in, and the end plate depolarises.
4. The depolarisation is large enough to set off an action potential in the sarcolemma, which sweeps across the fibre and down the T tubules.
5. In the T tubule the voltage change is sensed and passed to the sarcoplasmic reticulum, which dumps stored calcium into the cytoplasm around the myofibrils.

Acetylcholine does not linger. The enzyme acetylcholinesterase, sitting in the cleft, chops it within milliseconds, which is why one nerve impulse gives one twitch rather than a permanent contraction. Poisons at this junction are instructive precisely because each blocks one step: curare occupies the end plate receptors so acetylcholine cannot act and the muscle goes limp, while organophosphate insecticides and nerve agents block acetylcholinesterase so acetylcholine accumulates and the muscle cannot stop firing.

The cross-bridge cycle, one turn at a time

Calcium is now in the cytoplasm, and the cycle can run. Follow one myosin head.

1. The switch. At rest, tropomyosin lies along the thin filament covering the places myosin would bind. Calcium binds troponin; troponin changes shape and drags tropomyosin aside; the binding sites are exposed.
2. Attachment. The myosin head, already cocked and carrying the products of a previous ATP split, binds actin. This attachment is the cross-bridge.
3. The power stroke. Phosphate and then ADP leave the head, and the head pivots toward the centre of the sarcomere, dragging the thin filament with it. Distance moved: a few nanometres. This is the only step that does mechanical work.
4. Release. A fresh ATP binds the myosin head, and that binding, not its splitting, breaks the bond to actin.
5. Re-cocking. The head splits that ATP into ADP and phosphate and uses the energy to swing back into the cocked position, ready to bind further along the thin filament.

The cycle repeats tens of times a second while calcium remains high, each head grabbing, pulling, letting go and grabbing again, like a rope team hauling hand over hand. Relaxation is not passive either: calcium pumps in the sarcoplasmic reticulum membrane spend ATP driving calcium back into store, tropomyosin slides back over the binding sites, and the filaments slide apart under whatever load is on them.

Back to the stiff jaw

Now answer the opening question. After death, membrane pumps fail and calcium leaks out of the sarcoplasmic reticulum into the cytoplasm, so tropomyosin uncovers the binding sites and myosin heads attach. Glycogen runs out, ATP production stops, and step 4 cannot happen: without a fresh ATP the head has nothing to make it let go. Every cross-bridge in the body locks. Rigor mortis ends only when decomposition enzymes destroy the proteins themselves. The core of it: ATP is needed to release a cross-bridge, not only to form one, which is why no energy means locked rather than loose.

Motor units, twitches, and two kinds of fibre

One motor neuron does not supply one fibre. It branches and supplies many, and the neuron plus all the fibres it commands is a motor unit. Every fibre in a unit fires together, so the unit is the smallest amount of force the nervous system can switch on. In the muscles that move the eye, one neuron may control fewer than ten fibres, giving extremely fine control. In the quadriceps one neuron may control a thousand, giving power at the cost of precision. To lift something heavier, the nervous system recruits more units, smallest first.

A single impulse gives a single twitch lasting tens of milliseconds. Impulses arriving faster than the fibre can relax add on top of each other, and above about 50 impulses a second the twitches fuse into a smooth sustained contraction called tetanus, which is what almost every voluntary movement actually is. Fibres also differ in how they pay for it.

Named muscles, and how the names give the answer away

Muscle names are descriptions. Seven clues cover almost all of them: location, shape, size, fibre direction, number of heads, attachments, and action. Deltoid is a triangle, from the Greek letter delta. Gluteus maximus is the largest of three gluteal muscles. Rectus abdominis runs straight up the abdomen, while the obliques run at an angle. Biceps brachii has two heads on the arm, triceps three. Sternocleidomastoid runs from the sternum and clavicle to the mastoid process behind the ear, which is its whole job description. Extensor digitorum extends the fingers.

Notice how they come in opposing pairs. Biceps flexes the elbow, triceps extends it; quadriceps extends the knee, hamstrings flex it. A muscle can only pull, never push, so every joint needs an antagonist to undo what the agonist did.

When the shock absorber is missing

Every contraction puts strain on the sarcolemma, and a protein called dystrophin anchors the contractile machinery inside the fibre to the membrane and, through it, to the surrounding tissue. Without dystrophin the membrane tears a little on every contraction. Calcium leaks in through the tears, enzymes are activated, fibres die, and over years muscle is replaced by fat and fibrous tissue.

The DMD gene that codes for dystrophin sits on the X chromosome, and the fault is recessive, so boys with one affected X have no second copy to fall back on. Duchenne muscular dystrophy and the milder Becker form together affect about 1 in 3,500 to 5,000 newborn males worldwide. In Duchenne, weakness appears in early childhood and worsens quickly: a toddler who climbs up his own legs to stand, because his hip extensors are too weak to do it directly, calves that look enlarged because dead muscle has been replaced by fat and fibrous tissue, blood creatine kinase far above normal because damaged fibres are leaking a muscle enzyme into the blood. Children are usually wheelchair-dependent by adolescence, and with current care many live into their twenties or beyond.

There is no cure. Corticosteroids slow the loss of strength and delay the loss of walking. Careful heart and breathing care matters because the same protein is missing from cardiac muscle and the diaphragm, and these are what usually determine survival. Newer antisense treatments aim at the reading frame itself: for certain mutations, a drug makes the cell skip the damaged section when it reads the gene, producing a shortened but partly working dystrophin, which is closer to converting Duchenne into Becker than to a cure.

Common misconceptions

• Muscle filaments shorten when a muscle contracts. They slide. The A band, which is exactly as wide as the thick filaments, does not change width during contraction, which is how this was settled.
• Relaxation is what happens when you stop spending energy. Pumping calcium back into the sarcoplasmic reticulum costs ATP, and releasing each cross-bridge costs another. A muscle with no ATP cannot relax at all.
• Lifting heavier weights means each fibre pulls harder. A fibre contracts fully or not at all. More force means more motor units recruited and each firing faster.

What to carry forward

A muscle is fascicles of multinucleate fibres, each packed with myofibrils made of sarcomeres. Thin actin filaments anchor at the Z discs, thick myosin filaments sit in the middle, and shortening happens by sliding, which is why the A band never changes width. A motor neuron releases acetylcholine at the end plate, an action potential runs along the sarcolemma and down the T tubules, and the sarcoplasmic reticulum releases calcium. Calcium moves troponin and tropomyosin off the binding sites, myosin attaches, the power stroke pulls, a new ATP releases the head and a split ATP re-cocks it. Motor units set how finely force can be graded, and fibre type sets how long it can be held. Dystrophin keeps the sarcolemma from tearing under all of this, and its absence is Duchenne muscular dystrophy. Remember: the muscle that cannot get ATP is not the muscle that cannot contract, it is the muscle that cannot let go.

Sources

1. OpenStax. (2022). 10.2 Skeletal muscle. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 10.3 Muscle fiber contraction and relaxation. In Anatomy and Physiology 2e. Rice University. openstax.org
3. MedlinePlus Genetics. (2022). Duchenne and Becker muscular dystrophy. National Library of Medicine. MedlinePlus
4. National Institute of Neurological Disorders and Stroke. (2023). Muscular dystrophy. National Institutes of Health. NINDS

The squid that settled it

In 1939 Alan Hodgkin and Andrew Huxley threaded a fine glass electrode down the inside of a squid's giant axon, a nerve fibre so thick, close to a millimetre across, that this was physically possible. They expected an impulse to wipe out the resting voltage difference across the membrane. Instead they found that during an impulse the inside of the axon overshot zero and went positive, then swung back. Thirteen years later they published a set of equations describing exactly which ions moved and when, work that won a share of the Nobel Prize in Physiology or Medicine in 1963.

The reason that overshoot matters is that it rules out the obvious explanation. A signal that merely discharged the membrane would stop at zero. Going past zero means something is actively carrying positive charge inward, and that turns out to be the whole trick.

What a neuron looks like

A neuron has three regions with three jobs. Dendrites are branched processes that receive signals, often thousands of them. The cell body or soma holds the nucleus and does the housekeeping. The axon is one long process that carries the output, and at its far end it splits into terminals that contact the next cell. Where the axon leaves the cell body there is a short stretch called the axon hillock, and this is where the decision to fire is taken, because it carries the highest density of the channels that start an impulse.

Sorted by direction, neurons come in three kinds: sensory neurons carrying information inward from receptors, motor neurons carrying commands outward to muscles and glands, and interneurons, which sit between the two and make up the vast majority of the cells in the brain and spinal cord.

Neurons are outnumbered by glia, the supporting cells, and glia are not passive packing.

Why a resting neuron is negative inside

At rest the inside of a neuron sits at about negative 70 millivolts relative to the outside. Three things produce that.

First, the concentrations are uneven: sodium, Na⁺, is roughly ten times more concentrated outside the cell, while potassium, K⁺, is more concentrated inside. Second, the membrane at rest leaks potassium far more readily than sodium, so potassium drifts out down its gradient and leaves unmatched negative charge behind. Third, the sodium-potassium pump spends ATP to push three Na⁺ out for every two K⁺ it brings in, which both maintains the gradients and exports one net positive charge per cycle.

Why this matters: the resting potential is not a battery waiting to be used up. It is a gradient continuously rebuilt at metabolic cost, which is why the brain, about two percent of body mass, consumes roughly a fifth of the body's resting energy.

One action potential, millisecond by millisecond

Signals arriving on the dendrites produce small local voltage changes called graded potentials, which fade with distance and can add together. If enough of them reach the axon hillock at once and push the membrane from negative 70 up to about negative 55 millivolts, threshold is crossed and the sequence below runs by itself.

1. Depolarisation. Voltage-gated sodium channels snap open. Na⁺ pours in down both its concentration gradient and the electrical gradient, driving the membrane potential up through zero to about positive 30 millivolts. That is the overshoot Hodgkin and Huxley saw.
2. Inactivation. Each sodium channel has a second gate that swings shut a fraction of a millisecond after the first one opened, regardless of voltage. Sodium entry stops.
3. Repolarisation. Voltage-gated potassium channels, which opened slowly, are now fully open. K⁺ leaves, carrying positive charge out, and the potential falls back down.
4. Undershoot. Those potassium channels are slow to close, so a little too much potassium leaves and the membrane dips below negative 70 before settling. The pump restores the gradients over the following milliseconds.

Total elapsed time: about two milliseconds. Two consequences follow. The absolute refractory period is the window when sodium inactivation gates are shut and no stimulus of any size can fire the cell again, which is what stops an impulse running backwards. The relative refractory period follows, when a stronger than usual stimulus can fire the cell. And because the sequence either happens completely or not at all, an action potential is all or none: a stronger stimulus does not make a bigger impulse, it makes more impulses per second. Intensity is coded as frequency.

If you sketch this as a graph with time on the horizontal axis and millivolts on the vertical, it looks like a narrow spike: a flat line at negative 70, a slow creep up to negative 55, then a near-vertical rise to positive 30, a slightly less steep fall through zero and past negative 70, and a shallow dip that drifts back up to the resting line.

Why myelin makes it a hundred times faster

An unmyelinated axon has to regenerate the whole action potential at every point along its length, which is slow. A myelinated axon is wrapped in fatty insulation broken by bare gaps about a micrometre wide, the nodes of Ranvier, and the voltage-gated sodium channels are concentrated at those nodes. Current flows passively under the insulated stretch and the impulse is regenerated only at each node, so it appears to leap along. That is saltatory conduction, from the Latin saltare, to jump.

The bottom two rows explain something you have felt. Stub your toe and there are two waves of pain: a sharp one almost at once, carried by A delta fibres, and a dull throb a second or so later, carried by C fibres crawling up the same leg at walking pace.

Across the gap

At the end of the axon the signal has to cross a synapse, and the mechanism is the one from the neuromuscular junction generalised. The impulse opens calcium channels in the terminal, calcium entry triggers vesicles to release neurotransmitter into the cleft, and the transmitter binds receptors on the next cell.

What differs from muscle is that the answer is not always yes. An excitatory synapse depolarises the next cell toward threshold; an inhibitory one, typically using GABA or glycine, opens channels that hold it further away from threshold. A single neuron may receive thousands of both, and it adds them up: spatial summation across many synapses at once, temporal summation across signals arriving in quick succession at one synapse. Only if the total at the axon hillock reaches negative 55 does the cell fire. Transmitter is then cleared fast, either broken down by enzymes or pumped back into the terminal, which is where many psychiatric and neurological drugs act. Glutamate is the main excitatory transmitter in the brain, GABA the main inhibitory one; acetylcholine, dopamine, serotonin and noradrenaline each have their own circuits.

Multiple sclerosis: insulation stripped

In multiple sclerosis the immune system attacks myelin in the brain, spinal cord and optic nerves. Where a patch of myelin is destroyed, saltatory conduction fails: the impulse slows, and in a badly damaged patch it stops. Symptoms therefore depend entirely on which tracts the patches sit in, which is why no two patients present alike. A common first episode is optic neuritis, subacute loss of central vision in one eye with pain on moving it, because the optic nerve is a favoured site.

The condition affects roughly 2.5 million people worldwide and about 1 in 1000 people of European ancestry, is about three times more common in women than men, and usually begins between ages 20 and 40. Between 70 and 80 percent of patients start with a relapsing-remitting course: attacks lasting days to weeks, then partial or complete recovery as inflammation settles and some remyelination occurs. Around 15 to 20 percent have a primary progressive course that worsens steadily from the start. Diagnosis uses the McDonald criteria, which ask for evidence that lesions are separated in space, in more than one part of the central nervous system, and in time, arising on more than one occasion, usually shown by MRI.

There is no cure, but disease-modifying therapies, among them interferon beta, glatiramer acetate, dimethyl fumarate, fingolimod and natalizumab, reduce how often relapses occur and how many new lesions appear. They act on the immune attack rather than on the myelin, which is the honest limit of current treatment.

Common misconceptions

• A stronger stimulus makes a bigger nerve impulse. Above threshold, every action potential in a given axon is the same size. Strength is coded by firing rate and by how many fibres are recruited.
• Nerve signals are electricity running along a wire. Nothing flows along the axon the way current flows down copper. Ions cross the membrane sideways, and the disturbance regenerates itself along the length.
• Myelin speeds signals by conducting better. Myelin is an insulator. It works by preventing charge leaking out, so the signal only has to be rebuilt at the nodes.

Looking back

A neuron receives on its dendrites, decides at its axon hillock and transmits along its axon. At rest the inside sits near negative 70 millivolts, held there by unequal ion concentrations, a membrane that leaks potassium more than sodium, and a pump exporting three sodium for every two potassium it imports. Reach negative 55 and voltage-gated sodium channels open, driving the membrane to positive 30 before they inactivate and potassium channels bring it back down, all in about two milliseconds. Refractory periods keep the impulse travelling one way, and the all-or-none rule forces intensity to be coded as frequency. Myelin with nodes of Ranvier converts a metre per second into a hundred, and synapses add excitatory and inhibitory inputs before the next cell decides anything. The point: multiple sclerosis is a disease of that insulation rather than of the nerve cells themselves, which is why its symptoms map onto the location of the damage and not onto any single function.

Sources

1. OpenStax. (2022). 12.4 The action potential. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 12.2 Nervous tissue. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Tafti, D., Ehsan, M., and Xixis, K. L. (2024). Multiple sclerosis. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. Hodgkin, A. L., and Huxley, A. F. (1952). A quantitative description of membrane current and its application to conduction and excitation in nerve. The Journal of Physiology, 117, 500-544.

The brain that could say one syllable

In 1861 Paul Broca presented to a Paris scientific society the brain of a man who, for more than twenty years, had been able to produce only one syllable however hard he tried. He understood what was said to him. He could gesture, follow instructions and use his right hand until late in his illness. What he could not do was assemble words. On the surface of that brain, in the lower part of the frontal lobe on the left, was a destroyed region.

One patient does not prove a theory, and Broca went on to collect more. But the case made a claim that had been argued about for fifty years suddenly testable: the brain is not a uniform organ that thinks with all of itself. Particular jobs live in particular places, and if you know the map you can work backwards from a lost function to a damaged region. Nearly all clinical neurology still runs on that idea.

The map, from the outside in

The nervous system splits into the central nervous system, brain and spinal cord, and the peripheral nervous system, every nerve outside them. Inside the central system, grey matter is where cell bodies and synapses sit, and white matter is bundles of myelinated axons, white because of the fat in the myelin. In the brain, grey matter is on the outside as the cerebral cortex and white matter underneath. In the spinal cord the arrangement is inverted.

The cortex is folded into ridges called gyri and grooves called sulci, which packs a large sheet into a small skull. One groove, the central sulcus, runs down each side of the brain and is the landmark everything else is described from.

Two strips of cortex face each other across the central sulcus. In front of it, the precentral gyrus is the primary motor cortex, from which commands to skeletal muscle leave. Behind it, the postcentral gyrus is the primary somatosensory cortex, where touch and position arrive. Both are mapped body part by body part, but not to scale: the area given to the hand and the lips is enormous and the area given to the trunk is small, because cortical area tracks how finely a part is controlled and how densely it is innervated, not how big it is. Draw a body with those proportions and you get the distorted figure called a homunculus, with vast hands and a tiny back.

Broca's area, in the inferior frontal gyrus of the dominant hemisphere, usually the left, assembles speech. Damage there gives effortful, telegraphic output from someone who still understands: the patient knows the word and cannot get it out. Wernicke's area, in the temporal lobe, handles comprehension, and damage there gives the opposite pattern, fluent speech that carries little meaning. The two hemispheres exchange traffic through the corpus callosum, a thick band of about 200 million axons.

Below the cortex

Under the cerebrum sit the structures that keep you alive whether or not you are thinking. The thalamus is the relay through which nearly all sensory traffic passes on its way to the cortex, smell being the exception. The hypothalamus, already met as the body's thermostat, runs the autonomic nervous system and commands the pituitary.

The brainstem has three parts stacked downward: the midbrain, involved in reflex responses to sight and sound; the pons, a bridge carrying fibres to the cerebellum and contributing to breathing control; and the medulla oblongata, which sets heart rate, blood vessel tone and breathing rhythm. A blow that damages the medulla is fatal in a way that damage to a large area of cortex is not.

Behind the brainstem is the cerebellum, about a tenth of the brain's mass but holding a large share of its neurons. It compares the movement you intended with the movement you are making and corrects the difference. Cerebellar damage does not cause weakness; it causes clumsiness, a wide unsteady walk and a tremor that appears only as the hand approaches its target.

Protection, plumbing and supply

Three membranes, the meninges, wrap the brain and cord: tough dura mater outermost, web-like arachnoid mater beneath it, and delicate pia mater bonded to the surface itself. In the space under the arachnoid runs cerebrospinal fluid, made in the four ventricles inside the brain and circulating around the whole central nervous system. It cushions, removes waste and makes the brain effectively buoyant, so it does not crush itself under its own weight. Meningitis is inflammation of those membranes, and a lumbar puncture samples that fluid from the lower back, below where the cord ends.

Supply is the vulnerable part. The brain is about two percent of body mass and takes roughly a fifth of the body's resting oxygen, and it stores almost no fuel. Blood arrives through two internal carotid arteries in front and two vertebral arteries behind, which join in a ring at the base of the brain, the circle of Willis, so that a blockage in one feeding artery can sometimes be bypassed. Between blood and brain tissue sits the blood brain barrier: capillary walls sealed by tight junctions and supported by astrocytes, which admits oxygen, carbon dioxide and glucose while excluding most large molecules and many drugs.

The spinal cord and its roots

The cord runs from the medulla down to about the first or second lumbar vertebra, and gives off 31 pairs of spinal nerves. In cross section its grey matter forms a butterfly, and the wings have jobs. The posterior or dorsal horns receive sensory input; the anterior or ventral horns hold the motor neurons whose axons go out to muscle. Around the butterfly, white matter columns carry ascending sensory tracts and descending motor tracts.

Each spinal nerve joins the cord by two roots, and the division is absolute. The dorsal root carries sensory fibres in, and their cell bodies sit just outside the cord in a swelling called the dorsal root ganglion. The ventral root carries motor fibres out. Cut one and you lose sensation in a strip of skin; cut the other and you lose movement while sensation remains.

The reflex arc, in five parts

A reflex is a response that happens without a decision, and every one has the same five components: a receptor, a sensory neuron, an integration centre in the cord or brainstem, a motor neuron, and an effector.

Work the simplest example. Tap the tendon below the kneecap and you stretch the quadriceps very slightly. Stretch receptors inside the muscle, called muscle spindles, fire. Their type Ia sensory fibres, the fastest in the body at 80 to 120 metres per second, run to the cord and synapse directly onto the alpha motor neurons of that same quadriceps. Those neurons fire and the muscle contracts, the leg kicks. One synapse in the whole loop, which is why the patellar reflex is called monosynaptic and why it is so fast: about 50 milliseconds from tap to kick. The brain is told what happened, but it is told afterwards.

A withdrawal reflex is more elaborate. Touch something hot and pain fibres excite interneurons in the cord, which excite the flexors of that limb and, at the same time, inhibit its extensors, so nothing fights the movement. In the other leg the opposite happens, extensors on and flexors off, so you do not fall over when the burnt foot lifts. That is the crossed extensor reflex, and it is polysynaptic, involving several interneurons.

What matters here: reflexes are tested clinically because they are a working circuit with a known route. An absent knee jerk points to a break somewhere in that specific loop; an exaggerated one points to loss of the descending inhibition the brain normally applies to it.

Crossing over

Descending motor fibres from the precentral gyrus travel down and, in the medulla, most of them cross to the opposite side in a bundle called the pyramidal decussation. Ascending sensory tracts cross too, at one level or another. The consequence is simple and clinically decisive: each hemisphere controls and feels the opposite side of the body.

Stroke: the clock is the treatment

A stroke is a sudden loss of brain function caused by a failure of blood supply. About 87 percent are ischaemic, a vessel blocked by a clot; the rest are haemorrhagic, a vessel bleeding into or around the brain. Because the brain stores no fuel, neurons in the core of a blocked territory begin to die within minutes. Around that core lies the penumbra, tissue that is starved but not yet dead, kept alive on trickle flow from neighbouring vessels. The whole of stroke treatment is a race to save the penumbra.

Now put the anatomy together. A clot in the left middle cerebral artery starves the left motor and sensory strips and, usually, Broca's area. The result is weakness and numbness on the right side, because the fibres crossed in the medulla, plus effortful speech. If it is the face and arm that are worst affected rather than the leg, that also fits, because the artery supplies the part of the strip where face and hand are represented. A clinician can predict the blocked vessel from the pattern of deficit before any scan.

Recognition is taught as FAST: Face drooping on one side, Arm that drifts down when both are held up, Speech slurred or muddled, Time to call emergency services at once. The reason for the urgency is the treatment windows. Intravenous thrombolysis, a drug that dissolves the clot, is given within 3 hours of symptom onset as standard and up to 4.5 hours in selected patients. Mechanical thrombectomy, in which a catheter is threaded up to the brain and the clot pulled out, is used within 6 hours for a large vessel blockage, and in carefully selected patients with the right imaging up to 16 or even 24 hours from when they were last known to be well. After that, treatment is rehabilitation: intensive, repetitive practice that exploits the nervous system's capacity to recruit surviving circuits for jobs they did not previously do.

Common misconceptions

• People use only ten percent of their brains. Functional imaging shows activity across the whole cortex over the course of ordinary tasks, and damage anywhere in it produces a deficit. The claim has no clinical or imaging basis.
• Reflexes are decisions made very fast. The knee jerk does not reach the brain before the leg moves. The circuit closes in the spinal cord, and the brain is informed after the event.
• A stroke means the brain has run out of oxygen everywhere. A stroke is local. Which functions are lost depends on which artery is blocked and what that artery supplies.

What you now know

The cortex is mapped: frontal for movement and planning, parietal for touch, temporal for hearing and language comprehension, occipital for vision, with the motor and sensory strips facing each other across the central sulcus and scaled by precision rather than size. The thalamus relays, the hypothalamus regulates, the brainstem runs breathing and circulation, and the cerebellum corrects movement in flight. Meninges and cerebrospinal fluid protect, and the blood brain barrier filters. In the cord, sensory fibres enter at the dorsal root and motor fibres leave at the ventral root, and a reflex arc closes locally through five components. Motor and sensory tracts cross, so each hemisphere serves the opposite side. The upshot: that crossing is why a left-sided clot produces right-sided weakness, and why FAST, applied within the first hours, is the single thing that most changes the outcome.

Sources

1. OpenStax. (2022). 13.2 The central nervous system. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 13.3 Circulation and the central nervous system. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Lui, F., Khan Suheb, M. Z., and Patti, L. (2026). Ischemic stroke. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. MedlinePlus. (2024). Stroke. National Library of Medicine. MedlinePlus

Three numbers, and every colour you have ever seen

Your retina contains three kinds of cone, and their pigments absorb light most strongly at about 564, 534 and 420 nanometres: roughly yellow-green, green and violet-blue. Nothing in your eye measures wavelength directly. What reaches the brain is three numbers, the relative responses of the three cone types, and every shade you have ever named is a ratio between them. Turquoise is not a wavelength your eye detects; it is a pattern of three outputs.

That is the shape of both lessons in this file. The eye and the ear are not mystical. They are two organs solving physics problems, one with light and one with pressure waves, and both end up doing the same thing: converting an external physical quantity into a pattern of action potentials.

The eye as an optical instrument

The eyeball has three coats. The outer fibrous layer is the tough white sclera, which becomes the transparent cornea at the front. The middle vascular layer is the choroid, dark with pigment and rich in vessels, continuing forward as the ciliary body and the iris. The inner neural layer is the retina. In front of the lens sits watery aqueous humour, constantly made and drained; behind it, the gel-like vitreous humour, which is made once and never replaced, so the specks that drift across your vision are permanent debris in it.

Most people assume the lens does the focusing. It does not do most of it. Light bends most sharply when it passes from air into a denser medium, and that happens at the cornea, which accounts for roughly two thirds of the eye's total refractive power. The cornea is a fixed lens. The internal lens supplies the adjustable remainder, and the iris in front of it acts as an aperture, its pupil narrowing in bright light and widening in dim.

Focusing on something close

Hold this text at arm's length and then bring it slowly toward your face. At some point it blurs. The adjustment you were making up to that point is accommodation, and the mechanism is counter-intuitive.

The lens is held by a ring of fibres, the suspensory ligaments, attached to the ring-shaped ciliary muscle. When the ciliary muscle relaxes, its ring is wide, the ligaments are taut, and they pull the lens flat: this is the setting for distance. When the ciliary muscle contracts, its ring narrows, the ligaments slacken, and the elastic lens springs into a rounder shape with more refractive power: this is the setting for near work. So reading is the active state, which is part of why close work for hours is tiring.

So what?: the lens stiffens with age and can no longer round up as far, which is presbyopia, and it is why reading glasses become near-universal from roughly the mid-forties whatever your distance vision has been.

The other two errors are about the length of the eyeball rather than the lens. In myopia, short sight, the eyeball is too long for its optics, so parallel light from a distant object focuses in front of the retina and distant things blur; a diverging, concave lens pushes the focus back. In hyperopia the eyeball is too short and the focus falls behind the retina; a converging, convex lens pulls it forward. Astigmatism is a cornea curved more steeply in one direction than another, so there is no single focal point at all.

The retina, built back to front

The retina is arranged in a way no engineer would choose. Light entering the eye passes through the layers of nerve cells first and only then reaches the photoreceptors at the very back, which face away from the light. The signal then travels forward again through those layers and leaves through the optic nerve.

Two special places. The fovea is a tiny pit at the centre of the retina, packed with cones and stripped of the overlying cell layers so light reaches them directly. It is where your sharpest vision lives, and it is why you move your eyes rather than your attention when you read. The optic disc, where the optic nerve leaves and the vessels enter, has no photoreceptors at all, so there is a genuine hole in each visual field. You never notice it because the two eyes cover for each other and the brain fills the rest.

Transduction runs on one chemical trick. A rod's pigment, rhodopsin, is a protein wrapped around a molecule derived from vitamin A. A photon strikes it, the small molecule changes shape, and that shape change starts a cascade that alters the cell's membrane potential. The pigment must then be regenerated, which takes time; that is dark adaptation, and it is why stepping from sunlight into a cinema leaves you blind for a minute and why severe vitamin A deficiency causes night blindness first.

The commonest inherited variation here is red-green colour blindness, in which one of the two longer-wavelength cone pigments is absent or shifted. The genes sit on the X chromosome, so the pattern matches Duchenne muscular dystrophy from an earlier lesson: far commoner in males, who have only one X to work with.

The ear: turning air pressure into a nerve signal

Sound is a travelling pattern of pressure in air. The problem the ear solves is that the inner ear is full of fluid, and pressure waves in air mostly bounce off a liquid surface rather than entering it. Shouting at a swimming pool does not move much water.

The outer ear collects: the pinna funnels sound into the ear canal, which ends at the eardrum, the tympanic membrane. The middle ear is an air-filled cavity spanned by three ossicles, the smallest bones you own: malleus, incus and stapes, Latin for hammer, anvil and stirrup, in that order from eardrum inward. The stapes presses on the oval window, the membrane-covered doorway to the fluid-filled inner ear.

Those three bones exist to fix the mismatch, and they do it in two ways at once. The eardrum has many times the area of the oval window, so the same total force is concentrated into a much smaller patch, raising pressure. And the ossicles act as a lever that trades movement for force. Together they amplify the pressure enough that most of the sound energy actually enters the fluid instead of reflecting away. The Eustachian tube runs from the middle ear to the throat and exists only to equalise pressure across the eardrum, which is why swallowing clears your ears as a plane descends and why a blocked tube after a cold makes hearing dull.

Inside the cochlea

The cochlea is a fluid-filled tube coiled like a snail shell. Running along its length is the basilar membrane, and that membrane is not uniform: near the oval window, at the base, it is narrow and stiff; at the far end, the apex, it is wide and floppy. Stiff and narrow resonates at high frequency, wide and floppy at low, exactly as a short tight guitar string sounds higher than a long loose one.

So a pressure wave entering at the oval window makes the membrane vibrate maximally at one position determined by the frequency of the sound. Frequencies near 20 kHz, the top of the human range, peak at the base. Frequencies near 20 Hz peak at the apex. The cochlea is a mechanical frequency analyser, and its map of place to pitch is called tonotopy; the same map is preserved all the way up into the auditory cortex.

Sitting on the basilar membrane is the organ of Corti, carrying rows of hair cells. Each has a bundle of fine stiff projections, the stereocilia, whose tips are tethered to each other by tiny filaments. When the membrane moves, the bundle bends, the tethers pull, and ion channels are levered open mechanically. No enzyme cascade, no second messenger: the channel is pulled open by a physical tug, which is why hearing can follow frequencies far too fast for chemistry.

Reading an audiogram

Pure tone audiometry tests one ear at a time with tones at set frequencies, typically the octaves and half octaves from 250 Hz to 8000 Hz, and finds the quietest level you reliably detect at each. Results are in decibels hearing level, dB HL, a scale set so that 0 dB HL is the average threshold of healthy young ears at that frequency. The zero point therefore shifts with frequency, because the ear is not equally sensitive across its range.

The band between moderate and severe is labelled moderately severe. Testing is done twice: by air conduction, through headphones, which tests the whole pathway, and by bone conduction, with a vibrator on the bone behind the ear, which bypasses outer and middle ear and drives the cochlea directly. Comparing them localises the fault. If bone conduction is normal but air conduction is at least 15 dB worse at some frequency, the problem lies in the outer or middle ear: a conductive loss. If air and bone thresholds sit within 10 dB of each other and both are raised, the problem is in the cochlea or the nerve: a sensorineural loss.

Work one. A seventeen-year-old who plays bass in a band tests at 10, 10, 15 and 20 dB HL at 250, 500, 1000 and 2000 Hz, then 45 dB HL at 4000 Hz, then 30 dB HL at 8000 Hz. Bone conduction matches air at every frequency. Read it in order: normal through 2000 Hz, a moderate loss at 4000 Hz, partial recovery at 8000. Bone matching air makes it sensorineural. And the shape is the giveaway, a dip centred at 4000 Hz with better hearing either side, which is the classic notch of noise damage. Speech would still sound mostly normal, because most speech energy sits below 4000 Hz, which is exactly why this loss goes unnoticed for years.

Why that notch never fills in

Sounds at or below 70 A-weighted decibels are unlikely to damage hearing however long you listen. Long or repeated exposure at or above 85 dBA can cause loss, and louder sounds do it faster. The mechanism is blunt: excessive movement shears and kills hair cells and their stereocilia bundles.

Here is the part that matters. Birds and amphibians regrow cochlear hair cells. Humans do not. As the National Institute on Deafness and Other Communication Disorders puts it, in humans they are gone for good. A United States study estimated that somewhere between 10 and 40 million adults under 70 show audiogram features suggesting noise-induced loss. It is also the one form of hearing loss that is entirely preventable, by distance, duration or ear protection.

For severe and profound sensorineural loss there is a device that works around the missing cells entirely. A cochlear implant takes sound from an external microphone, splits it into frequency bands electronically, and delivers each band as current to an electrode threaded along the cochlea at the place that band would naturally have stimulated. It is not amplification; it substitutes for the hair cells by exploiting the tonotopic map directly.

Common misconceptions

• The lens does most of the focusing. The cornea supplies about two thirds of the eye's refractive power and cannot change. The lens supplies the adjustable part.
• The ciliary muscle contracts to see far away. The opposite. Contraction slackens the ligaments and lets the lens round up for near vision; relaxation stretches the lens flat for distance.
• A hearing loss means everything sounds quieter. Noise damage attacks particular frequencies first, usually around 4000 Hz, so speech can seem clear while consonants blur, especially against background noise.

Summing up

Light is bent chiefly at the cornea, fine-tuned by a lens that the ciliary muscle rounds up for near work, and lands on a retina wired back to front, with rods for dim light across the periphery and three cone types clustered at the fovea whose pigments peak near 564, 534 and 420 nanometres. The optic disc has no receptors and produces a genuine blind spot. Sound is funnelled to the eardrum, and three ossicles convert air vibration into fluid vibration by concentrating force onto the small oval window. The basilar membrane, stiff at the base and floppy at the apex, sorts frequency by place, and hair cells convert the movement into nerve signals by having their ion channels physically pulled open. An audiogram plots threshold against frequency, and comparing air with bone conduction says whether a loss is conductive or sensorineural. Worth holding on to: hair cells do not come back, so the 4000 Hz notch on an audiogram is a permanent record of what somebody listened to.

Sources

1. OpenStax. (2022). 14.1 Sensory perception. In Anatomy and Physiology 2e. Rice University. openstax.org
2. Carl, A. C., Hohman, M. H., and Cornejo, J. (2023). Audiology pure tone evaluation. In StatPearls. StatPearls Publishing. NCBI Bookshelf
3. National Institute on Deafness and Other Communication Disorders. (2022). Noise-induced hearing loss. National Institutes of Health. NIDCD
4. National Eye Institute. (2024). Nearsightedness (myopia). National Institutes of Health. NEI

Toronto, 11 January 1922

A thirteen-year-old boy called Leonard Thompson was given an injection of pancreatic extract in a Toronto hospital. He weighed about 30 kilograms and was close to death, because before that day a diagnosis of what we now call type 1 diabetes killed within months. The first extract was impure and produced an abscess with little benefit. A better-purified batch was given on 23 January, and over a two-week course of daily injections his blood glucose fell, the sugar disappeared from his urine, and he began to gain weight. He lived another thirteen years.

What that extract contained was one hormone, and the story is the cleanest demonstration of what a hormone is: a chemical made in one place, carried in the blood, acting somewhere else entirely, in quantities small enough that a few milligrams a day makes the difference between living and dying.

Two messaging systems, running at different speeds

The body has two ways of coordinating itself, and the differences are not arbitrary.

The last row is the important one. A hormone goes everywhere. What decides where it acts is not delivery but reception: only cells with the right receptor respond. This is why one gland can coordinate a response across a dozen tissues at once, and why endocrine disease so often produces a scattered list of symptoms.

Two chemistries, two mechanisms

Hormones come in two broad chemical classes, and the class predicts almost everything about how the hormone behaves.

Steroid hormones, built from cholesterol, include cortisol, aldosterone, oestrogen and testosterone. Being lipid-soluble, they pass straight through the cell membrane and bind receptors inside the cell, usually in the nucleus, where the hormone-receptor complex switches genes on or off. Effects therefore take hours, because proteins have to be made, and last a long time. Being lipid-soluble also means they need carrier proteins to travel in watery plasma.

Peptide and amine hormones, including insulin, glucagon, growth hormone and adrenaline, are water-soluble and cannot cross the membrane. They bind receptors on the cell surface, which trigger second messengers inside. Effects arrive in seconds to minutes because the machinery is already built and is merely being switched on. Thyroid hormone is the awkward exception: an amine that behaves like a steroid, crossing the membrane and acting on genes.

The gland that takes its orders from the brain

The pituitary gland hangs below the hypothalamus on a short stalk, and is really two glands stuck together.

The posterior pituitary is nervous tissue. It makes no hormones of its own; it stores two, antidiuretic hormone and oxytocin, that were manufactured by neurons in the hypothalamus and carried down their axons. Release happens when those neurons fire. Antidiuretic hormone tells the kidney to conserve water, and reappears in the urinary lesson; oxytocin drives labour contractions and milk ejection.

The anterior pituitary is glandular tissue and does make its own hormones, but only on instruction. The hypothalamus releases tiny quantities of releasing and inhibiting hormones into a private set of blood vessels, the hypophyseal portal system, running the few millimetres down to the anterior pituitary. The anterior pituitary then releases growth hormone, thyroid stimulating hormone, adrenocorticotropic hormone, follicle stimulating hormone, luteinising hormone and prolactin.

Key idea: most endocrine control is a three-tier chain. Hypothalamus signals pituitary, pituitary signals target gland, target gland releases its hormone, and that hormone feeds back to suppress both of the tiers above it. That is negative feedback again, this time in an organisational hierarchy rather than a thermostat.

The glands and what they release

Two of those pairs oppose each other directly, and both should look familiar: calcitonin against parathyroid hormone for calcium, which you met with bone, and insulin against glucagon for glucose, which is next.

Blood glucose, worked as a loop

A useful figure first. An adult carries roughly five litres of blood, and a normal fasting glucose concentration of about 90 milligrams per decilitre means 0.9 grams per litre, so the entire circulating glucose pool is about 4.5 grams, slightly more than a teaspoon. A sandwich supplies ten times that. The whole system exists to absorb that shock.

The normal fasting range is 99 milligrams per decilitre or below. Divide by 18 to convert to millimoles per litre, the unit used in much of the world, so 99 mg/dL is about 5.5 mmol/L.

After a meal. Glucose absorbed from the gut raises blood concentration. Beta cells in the pancreatic islets detect the rise directly and release insulin. Insulin does three things at once. In muscle and fat it causes glucose transporter proteins stored inside the cell to move into the membrane, so glucose enters cells that were previously nearly closed to it. In the liver it drives glucose into storage as glycogen. Everywhere it promotes building rather than breaking down. Blood glucose comes back into range, and insulin release falls off. That fall is the negative feedback.

Between meals. Glucose drifts down. Alpha cells in the same islets release glucagon, which acts almost entirely on the liver: break glycogen back down into glucose, and, when glycogen runs low, manufacture fresh glucose from amino acids and glycerol. Blood glucose returns to range and glucagon release falls off. If the fall is steep or fast, adrenaline and cortisol join in, which is why a hypoglycaemic episode feels like fear: shaking, sweating and a racing heart are the adrenaline, not the low sugar itself.

Notice that the two hormones come from neighbouring cells in the same microscopic islet, sampling the same blood. The pancreas is not consulting a distant sensor; the sensor and the effector are the same cell.

When the loop breaks, in two different ways

Both diseases called diabetes end with glucose too high. They arrive there by opposite routes.

Type 1 is an autoimmune disease, and it returns in the immunity lesson for that reason. Type 2 is a disease of receptor response: insulin is present, often at high levels for years, and the cells have stopped listening.

Diagnosis uses the same numbers for both.

The A1C test deserves a word, because its mechanism is elegant. Glucose in the blood attaches slowly and irreversibly to haemoglobin inside red cells, without any enzyme doing it. The higher the average glucose, the more haemoglobin gets tagged. Since a red cell lives about 120 days, the percentage of glycated haemoglobin reports average glucose over roughly the previous two to three months, and cannot be improved by behaving well for a week beforehand.

What treatment actually does

In type 1 the missing molecule is replaced. Modern regimens use a long-acting insulin for background needs plus rapid-acting doses matched to the carbohydrate in each meal, delivered by injection or a pump, with finger-prick meters or continuous glucose monitors closing the loop. The judgement the beta cell used to make automatically has to be made consciously, several times a day, for life.

In type 2 the aim is to make the existing insulin work. Weight loss, physical activity and dietary change improve insulin sensitivity directly, and exercise has its own route into muscle cells that does not need insulin at all, which is why a walk after a meal lowers glucose. Drug treatment usually begins with metformin, which reduces the liver's glucose output and improves sensitivity. Newer classes make the kidney excrete glucose in the urine, or mimic a gut hormone that amplifies insulin release after eating. Insulin itself is added when the beta cells can no longer meet demand.

The upshot: the same blood number can mean no key, or a key that no longer fits the lock, and the treatments follow from which.

Common misconceptions

• Diabetes is caused by eating sugar. Type 1 is an immune attack on the beta cells and has nothing to do with diet. Type 2 has dietary and weight-related risk factors, but no single food causes it and many people with the risk factors never develop it.
• Type 2 is the mild kind. It is the kind that develops more slowly and is often silent for years, which is why complications in the eye, kidney and nerves are frequently present at diagnosis.
• A hormone reaches only its target organ. It reaches every organ. Only cells carrying the right receptor can answer, which is why the same adrenaline speeds the heart and opens the airways.

What to remember

Hormones travel everywhere and act only where a receptor waits. Steroids cross the membrane, work on genes and act slowly and durably; peptides and amines work through surface receptors and act within minutes. The hypothalamus commands the anterior pituitary through a private portal circulation and stocks the posterior pituitary through axons, and most axes are three tiers deep with the final hormone suppressing the tiers above. Blood glucose is defended by insulin from beta cells, which moves transporters into membranes and fills the liver with glycogen, and glucagon from alpha cells, which empties it again. Type 1 destroys the beta cells and requires insulin; type 2, roughly 90 to 95 percent of cases, is a loss of response. The A1C test reads average glucose over about three months because glucose sticks permanently to haemoglobin inside a cell that lives 120 days. The point: the glucose loop is the same receptor, control centre and effector architecture as thermoregulation in lesson one, with hormones in place of nerves.

Sources

1. OpenStax. (2022). 17.9 The endocrine pancreas. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 17.3 The pituitary gland and hypothalamus. In Anatomy and Physiology 2e. Rice University. openstax.org
3. National Institute of Diabetes and Digestive and Kidney Diseases. (2023). Diabetes tests and diagnosis. National Institutes of Health. NIDDK
4. National Institute of Diabetes and Digestive and Kidney Diseases. (2023). What is diabetes? National Institutes of Health. NIDDK

Eight hundred milliseconds

One complete beat of a resting adult heart takes about 800 milliseconds, and the time is not shared out evenly. The atria contract for roughly 100 milliseconds. The ventricles contract for about 270. The remaining 430 milliseconds, more than half the cycle, everything is relaxed and filling. Your heart spends most of its life resting, in short instalments, which is the only reason a muscle can contract two and a half billion times without a day off.

This lesson follows one cycle through, then shows you the three ways that cycle can be observed from outside the body: a stethoscope, a blood pressure cuff and an electrocardiogram. Each one records a different part of the same 800 milliseconds.

What is actually being pumped

An adult carries roughly 5 to 6 litres of blood if male and 4 to 5 if female. Spin a sample in a centrifuge and it separates. About 45 percent settles as red cells, a figure called the haematocrit, with a normal range near 42 to 52 percent in men and 37 to 47 in women. A thin buffy coat of white cells and platelets, less than 1 percent of the sample, sits on top of them. The remaining 55 percent is plasma, itself about 92 percent water and 7 percent protein. Blood runs at about 38 degrees Celsius, slightly warmer than the rest of you, and its pH is held between 7.35 and 7.45.

Each haemoglobin molecule holds four iron-containing haem groups, and each binds one oxygen molecule, so one haemoglobin carries four. Binding is cooperative: the first oxygen makes the next easier to attach, which is why haemoglobin loads almost completely in the lung and yet still unloads readily in a working muscle.

Four chambers and a one-way system

The heart sits in the mediastinum, tilted so its apex points down and to the left, wrapped in the pericardium. Its wall is mostly myocardium, cardiac muscle, with the branched cells and intercalated discs you met in lesson two.

Right side and left side do different jobs. The right side receives oxygen-poor blood from the body and pushes it to the lungs: the pulmonary circuit, short and low pressure. The left side receives oxygenated blood from the lungs and pushes it to everything else: the systemic circuit, long and high pressure. The two sides beat together but are completely separated by the septum, and the left ventricular wall is roughly three times thicker than the right because of the pressure it must generate.

Trace one circuit. Blood returns from the body through the superior and inferior vena cava into the right atrium, passes the tricuspid valve into the right ventricle, leaves through the pulmonary valve into the pulmonary arteries, and reaches the lungs. It returns oxygenated through the pulmonary veins into the left atrium, passes the mitral valve into the left ventricle, and is ejected through the aortic valve into the aorta. Note the naming trap: pulmonary arteries carry deoxygenated blood and pulmonary veins carry oxygenated blood, because artery means away from the heart, not oxygen-rich.

The valves are passive. They open and shut because of pressure differences either side of them, not because anything pulls on them. The tricuspid and mitral valves are anchored by tendinous cords to small papillary muscles that tighten during contraction, not to pull the valve shut but to stop it turning inside out under 120 millimetres of mercury.

The cycle, in milliseconds

Start with everything relaxed. Blood is returning passively from the veins, flowing through the open atrioventricular valves and filling the ventricles. Most ventricular filling happens here, before the atria do anything.

1. Atrial systole, about 100 ms. The atria contract and top up the ventricles with the last portion. The ventricles now hold their end diastolic volume, about 130 millilitres in a resting adult.
2. Isovolumetric contraction. The ventricles begin to contract. Pressure inside rises above atrial pressure, so the atrioventricular valves slam shut, and that is the first heart sound, the lub. For a moment all four valves are closed and the volume cannot change, hence the name.
3. Ejection. Ventricular pressure passes aortic and pulmonary pressure, the semilunar valves open, and blood is ejected. Ventricular systole lasts about 270 ms in total. The volume ejected is the stroke volume, 70 to 80 millilitres, leaving 50 to 60 millilitres behind as end systolic volume. A ventricle never empties.
4. Isovolumetric relaxation. The ventricles relax, pressure inside falls below aortic pressure, and blood starting to fall back shuts the semilunar valves. That is the second heart sound, the dub.
5. Filling, about 430 ms. Pressure falls below atrial pressure, the atrioventricular valves open, and the cycle begins again.

Key idea: both heart sounds are valves closing, not chambers contracting, and each one marks the boundary between two phases. Multiply stroke volume by heart rate and you get cardiac output: 70 millilitres times 70 beats a minute is about 4.9 litres a minute, which means your entire blood volume passes through the heart roughly once a minute at rest.

Who tells it to beat

Cardiac muscle is myogenic: it generates its own rhythm without any nerve. The sinoatrial node, a patch of specialised cells in the wall of the right atrium, depolarises spontaneously faster than anything else in the heart and therefore sets the pace, typically 60 to 100 times a minute in a resting adult. Its impulse spreads across both atria, making them contract.

It cannot cross into the ventricles directly, because a ring of fibrous tissue insulates atria from ventricles. The only route is the atrioventricular node, which deliberately slows the signal by about a tenth of a second. That delay is functional: it lets the atria finish emptying before the ventricles start. From there the signal runs down the bundle of His, splits into left and right bundle branches, and spreads through Purkinje fibres to the ventricular muscle, which contracts from the apex upward, squeezing blood toward the outflow valves rather than sloshing it about.

Nerves modify but do not create the rhythm. Sympathetic fibres and adrenaline speed the node; parasympathetic fibres in the vagus nerve slow it. Cut every nerve to a transplanted heart and it still beats, a little faster than usual because the vagus is no longer applying the brakes.

Reading an ECG

An electrocardiogram records the summed electrical activity of all that muscle from electrodes on the skin. It shows depolarisation and repolarisation, not contraction, which is the single most useful thing to hold on to.

Atrial repolarisation happens too, but it is buried inside the much larger QRS complex and cannot be seen. Sketch the trace: a flat baseline, a small rounded bump upward (P), a flat stretch, a sharp narrow spike with a small dip either side (QRS), then a broader rounded bump (T), then flat again until the next P. Heart rate can be read straight off the spacing of the QRS spikes, and a rhythm that fires from somewhere other than the sinoatrial node announces itself by the P wave being absent, inverted or out of step.

What the cuff is measuring

A blood pressure reading gives two numbers in millimetres of mercury. The systolic figure is arterial pressure at the peak of ventricular ejection; the diastolic figure is the pressure the arteries never fall below between beats, which exists only because large arteries are elastic and recoil. A cuff inflated above systolic pressure stops flow entirely; as it deflates, blood begins forcing through in spurts, and those turbulent spurts are the Korotkoff sounds heard through a stethoscope. The pressure at which they start is systolic; the pressure at which they disappear, because flow has become smooth again, is diastolic.

Watch the word in the middle column. Stage 1 needs only one of the two numbers to qualify, which is why a reading of 118 over 84 is not normal.

When a coronary artery closes

The myocardium cannot take oxygen from the blood inside its chambers. It has its own supply, the coronary arteries, branching off the aorta just above the aortic valve. In atherosclerosis, fatty plaques build in those arteries over decades. A myocardial infarction typically begins when one of those plaques ruptures, exposing material that triggers clotting. A thrombus forms on the spot and blocks the artery, and the muscle downstream starts to die.

Three lines of evidence identify it. The story: crushing central chest pain, often radiating to jaw or left arm, with sweating, nausea and breathlessness, and frequently more vague in women, older people and people with diabetes. The ECG: elevation of the segment between the QRS and the T wave in two adjacent leads marks a full-thickness infarct, a STEMI, and its absence with other changes marks a NSTEMI. The blood test: cardiac troponin, the regulatory protein you met on the thin filament, leaks from dying cardiac muscle and is the most specific marker available, peaking around 12 hours and staying raised for about a week.

Treatment is reopening the artery, and speed is the whole of it. Primary percutaneous coronary intervention, threading a catheter to the blockage and inflating a balloon to crush the plaque aside, usually leaving a stent, is preferred to clot-dissolving drugs when it can be done within 120 minutes of the diagnostic ECG, and reperfusion is aimed at within 12 hours of symptom onset. Afterwards, the dead area heals as fibrous scar, which neither contracts nor conducts, and the long-term consequences follow from how much muscle was lost before flow returned.

Common misconceptions

• The heart sounds are the chambers contracting. Both are valves closing. The lub is the atrioventricular valves at the start of ventricular contraction and the dub is the semilunar valves at the end of it.
• Arteries carry oxygenated blood and veins carry deoxygenated blood. The pulmonary vessels are the other way round. Artery means away from the heart.
• An ECG shows the heart pumping. It shows electrical activity only. Contraction follows depolarisation, and in some emergencies the trace looks organised while the heart is moving no blood at all.

Where this leaves us

Blood is 45 percent red cells and 55 percent plasma, carrying oxygen on haemoglobin's four haem groups. The right heart serves the lungs at low pressure and the left heart serves the body at high pressure, with valves opened and closed by pressure alone. One cycle at rest takes 800 milliseconds: 100 of atrial contraction, 270 of ventricular contraction and 430 of filling, ejecting 70 to 80 millilitres per beat. The sinoatrial node sets the pace, the atrioventricular node inserts the delay that lets filling complete, and the ECG records that electrical sequence as P, QRS and T. A blood pressure reading is arterial pressure at its peak and its trough, and 130 or 80 is the current threshold for stage 1 high blood pressure on either number alone. Bottom line: a heart attack is a plumbing failure in the arteries that feed the pump itself, and every minute before reperfusion is muscle that will be replaced by scar.

Sources

1. OpenStax. (2022). 19.3 Cardiac cycle. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 18.1 An overview of blood. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Mechanic, O. J., Gavin, M., Shams, P., and Grossman, S. A. (2023). Acute myocardial infarction. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. National Heart, Lung, and Blood Institute. (2024). High blood pressure. National Institutes of Health. NHLBI
5. Whelton, P. K., et al. (2018). 2017 ACC/AHA/AAPA/ABC/ACPM/AGS/APhA/ASH/ASPC/NMA/PCNA guideline for the prevention, detection, evaluation, and management of high blood pressure in adults. Hypertension, 71, e13-e115.

Twenty-four litres out, twenty and a half back

Across all your capillaries, roughly 24 litres of fluid leaves the bloodstream every day and about 20.4 litres returns to it directly. The missing 3.6 litres does not stay in the tissues, and it does not vanish. A second, one-way drainage network collects it and empties it back into the veins at the base of the neck. If that network stops working in one limb, the limb swells and keeps swelling.

Most people meet the lymphatic system as a list of nodes to memorise. It is easier to understand as the answer to a leak, and to see the leak you first need the vessels it leaks from.

Three wall layers, three designs

Every vessel larger than a capillary has the same three layers. The tunica intima is a single sheet of flat endothelium on a thin base, and it is the layer that touches blood. The tunica media is smooth muscle and elastic fibres. The tunica externa is connective tissue anchoring the vessel to its surroundings. What differs between vessel types is the thickness of the middle layer, and every functional difference follows from that.

A capillary is about 8 micrometres wide, and a red blood cell is about 7.5, so red cells pass in single file, deforming as they go. That is not a design flaw. Single file means every red cell touches the wall, and the wall is one cell thick, so the diffusion distance for oxygen is as short as biology can make it.

Capillaries come in three grades of leakiness. Continuous capillaries have tightly joined endothelium and are the standard, at their tightest in the brain where they form the blood brain barrier. Fenestrated capillaries are punched with pores, found where rapid filtration is the job: the kidney's glomerulus, the intestinal lining, endocrine glands. Sinusoids have wide gaps and incomplete basement membrane, found in liver, spleen and bone marrow where whole cells and large proteins must cross.

Where the pressure goes

Blood leaves the left ventricle at about 120 millimetres of mercury and arrives back at the right atrium at essentially zero. Tracking where those 120 millimetres are spent tells you what each part of the circuit is for.

The aorta and large elastic arteries stretch during systole and recoil during diastole, which is what converts an intermittent pump into continuous flow and is why there is a diastolic pressure at all. Muscular arteries distribute. Then come the arterioles, and this is where most of the pressure is lost, because their muscular walls can constrict or dilate and they are where resistance is set. That makes arterioles the control valves of the whole system: the same total cardiac output can be sent to muscle or to gut depending on which arterioles are open.

In the veins the problem reverses. Pressure is now too low to push blood uphill from your feet, so veins get help. Valves allow flow only toward the heart. Contracting skeletal muscles squeeze the deep veins between them, the skeletal muscle pump, and each squeeze pushes blood past the next valve. Breathing helps too, since dropping the diaphragm lowers pressure in the chest and draws blood in. Stand still for a long time and both pumps stop working, blood pools in the legs, venous return falls, cardiac output falls, and people faint: which is exactly why standing still at attention makes soldiers drop, and why shifting your weight prevents it. Where valves fail permanently, blood pools and stretches the superficial veins into varicose veins.

The tug of war at the capillary wall

Two opposing pressures decide whether fluid leaves a capillary or enters it, and both act along the whole of its length.

Hydrostatic pressure is the physical push of blood against the wall. It is about 35 millimetres of mercury at the arterial end of a capillary and has fallen to about 18 by the venous end, because pressure is lost along the way. It pushes fluid out.

Colloid osmotic pressure is the pull created by plasma proteins, chiefly albumin, which are too large to leave. Water follows them. This pull is about 25 millimetres of mercury and, unlike hydrostatic pressure, does not change much along the capillary.

Now do the subtraction. At the arterial end, roughly 35 pushing out against 25 pulling in leaves a net outward pressure of about 10 millimetres of mercury, so fluid filters out into the tissue carrying dissolved oxygen, glucose and amino acids with it. At the venous end, roughly 18 pushing out against 25 pulling in gives a net inward figure of about minus 7, so fluid is reabsorbed, bringing carbon dioxide and waste back. What matters here: the same capillary leaks at its start and drinks at its finish, and the switch happens because hydrostatic pressure falls while osmotic pull does not.

The two sides do not balance exactly, and that is the 3.6 litre daily surplus. Note also that this bulk flow moves fluid, not most of the gases. Oxygen and carbon dioxide cross mainly by simple diffusion down their concentration gradients, and glucose and amino acids by facilitated transport through carriers, neither of which needs the pressure story at all.

The drain

The lymphatic system begins as blind-ended lymphatic capillaries sitting among the tissue cells. Their endothelial cells overlap like roof tiles, anchored to surrounding tissue by fine filaments. When interstitial fluid accumulates, the tissue swells slightly, the filaments pull the flaps open, and fluid enters. As pressure inside rises the flaps are pushed shut. It is a one-way valve made of overlapping cells, opened by the very pressure it is there to relieve.

Once inside, the fluid is called lymph. It travels through progressively larger lymphatic vessels, which have valves like veins and are squeezed by the same muscle and breathing pumps, since there is no pump of their own. On the way it passes through lymph nodes, small encapsulated masses packed with immune cells that filter it and sample it for anything worth reacting to, which is why nodes swell when you have an infection. Eventually lymph collects into two ducts: the thoracic duct, draining everything except the upper right quadrant of the body, and the right lymphatic duct, draining that quadrant. Both empty into the large veins beneath the collarbones, returning the fluid to the blood.

The same organ system does other jobs. Lacteals in the small intestine absorb digested fat and deliver it into lymph rather than blood, which is why the thoracic duct's contents look milky after a fatty meal. The spleen filters blood rather than lymph and removes worn-out red cells. The thymus, in the chest and largest in childhood, trains one class of lymphocyte, which the immunity lesson returns to. Tonsils and patches of tissue in the gut wall guard entry points.

Keeping the pressure right

Blood pressure is defended by its own negative feedback loop, and it is fast. Stretch receptors called baroreceptors sit in the wall of the carotid sinus and the arch of the aorta and fire in proportion to how much the wall is stretched. Their signals go to the cardiovascular centre in the medulla. If pressure rises, firing increases, the centre reduces sympathetic output and increases vagal output, so heart rate and force fall and arterioles dilate. If pressure falls, the opposite. Stand up quickly and the transient dizziness is the second or two before this loop catches up.

Slower control works through fluid volume. The kidney releases renin when its perfusion falls, starting a cascade that produces angiotensin II, a powerful vasoconstrictor, and triggers aldosterone from the adrenal cortex, which makes the kidney retain sodium and water. Antidiuretic hormone from the posterior pituitary conserves water directly. These take minutes to hours rather than seconds, and they are the targets of several of the commonest blood pressure drugs.

When the fluid stays out there

Oedema is visible swelling from fluid accumulating in the tissues, and the tug of war predicts every route to it. Raise capillary hydrostatic pressure, as heart failure does by backing pressure up into the veins, and more filters out than returns. Lower plasma protein, as severe liver disease or protein loss in the urine does, and the inward pull weakens. Increase capillary permeability, as inflammation does, and protein leaks into the tissue, taking its osmotic pull with it. Or block the drain.

Blocking the drain gives lymphoedema, and it is distinctive: swelling of one limb, often firm rather than pitting once it is established, that does not resolve with elevation. More than 90 percent of cases worldwide are secondary, acquired through damage or obstruction. In tropical and subtropical regions the leading cause is lymphatic filariasis, a parasitic worm that lodges in lymph vessels. In wealthier countries the leading cause is cancer treatment: after surgery to the armpit for breast cancer, somewhere between 6 and 30 percent of patients develop clinically significant arm lymphoedema, with the risk highest after full axillary node dissection with radiation and lowest after a sentinel node biopsy alone.

Treatment is mechanical, because the missing structure cannot be regrown. Complete decongestive therapy combines specialised manual lymphatic drainage, multilayer bandaging, exercise and skin care, first intensively and then as lifelong maintenance, with compression garments essential at every stage. Surgery, such as joining a lymph vessel to a small vein or transplanting nodes, is reserved for cases that do not respond.

Common misconceptions

• Veins are just arteries running the other way. Veins carry most of the body's blood at any moment, hold it at low pressure, collapse when empty and depend on valves and surrounding muscle to move it.
• Oxygen crosses into tissue by being pushed out with the filtered fluid. Bulk flow moves fluid. Gases cross by diffusion down their own gradients, independently of the pressure balance.
• Swollen lymph nodes mean cancer. Nodes swell because immune cells inside them are dividing in response to something they filtered, which is most often an ordinary infection.

Pulling it together

Arteries, capillaries and veins share three wall layers and differ in the thickness of the muscular middle one. Arterioles hold most of the resistance and therefore decide where blood goes; capillaries are one cell thick and about 8 micrometres wide so exchange happens over the shortest possible distance; veins are low pressure reservoirs that need valves and the skeletal muscle pump. At the capillary, hydrostatic pressure of about 35 millimetres of mercury falling to 18 competes with a steady colloid osmotic pull of about 25, so fluid leaves at the arterial end and returns at the venous end, with a surplus of a few litres a day. Lymphatic capillaries collect that surplus through overlapping flap valves, pass it through nodes, and return it to the veins under the collarbones. Baroreceptors defend pressure in seconds and the kidney defends it in hours. The upshot: every form of oedema is one of four things gone wrong in that tug of war, and which one it is determines the treatment.

Sources

1. OpenStax. (2022). 20.3 Capillary exchange. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 20.1 Structure and function of blood vessels. In Anatomy and Physiology 2e. Rice University. openstax.org
3. OpenStax. (2022). 21.1 Anatomy of the lymphatic and immune systems. In Anatomy and Physiology 2e. Rice University. openstax.org
4. Nimmana, B. K., Kimyaghalam, A., and Manna, B. (2025). Lymphedema. In StatPearls. StatPearls Publishing. NCBI Bookshelf

Six litres in, half a litre moving

An adult man's lungs hold about 6000 millilitres of air when filled completely, and a woman's about 4200. A quiet breath moves only about 500 of those millilitres, twelve to eighteen times a minute. And roughly 150 millilitres of that half litre never reaches a gas exchange surface at all; it sits in the nose, trachea and bronchi, gets breathed back out unchanged, and is called anatomical dead space. Breathing is a small tidal movement on top of a large standing volume, and that arrangement is what keeps the composition of alveolar air steady instead of swinging with every breath.

This lesson works from that air inward: down the branching airway, across the membrane, onto the haemoglobin, then to the control system that decides how fast you do it, and finally to what an asthma attack does to the pipes.

The airway, branch by branch

Air enters through the nose, where it is warmed, moistened and filtered by hairs and mucus, and passes the pharynx and the larynx. The epiglottis folds over the laryngeal opening during swallowing, which is the only thing keeping food out of the airway. Below the larynx, the trachea is held open by about twenty C-shaped rings of cartilage, incomplete at the back so the oesophagus behind it can bulge when you swallow.

The trachea divides into two main bronchi, which divide again, and again, roughly twenty-three times in all. As the tubes get smaller the cartilage disappears and smooth muscle takes over, which is what makes the smallest ones, the bronchioles, able to change diameter and therefore able to close. That single anatomical fact is the whole of asthma.

Everything from nose to terminal bronchiole is the conducting zone: it moves air and exchanges no gas. Its lining is ciliated epithelium with goblet cells, and the mucus escalator those cilia drive carries trapped particles up to the throat to be swallowed. Beyond that is the respiratory zone, ending in clusters of alveoli.

An alveolus is a thin-walled sac about a fifth of a millimetre across, wrapped in capillaries. Its wall has two cell types. Type I cells are extremely flattened squamous epithelium and form almost all of the surface, because gas has to cross them. Type II cells are scattered cuboidal cells that secrete surfactant, a detergent-like mixture that lowers the surface tension of the fluid lining the sac. Without surfactant the water film would pull each alveolus shut on every exhalation and reopening it would take enormous effort, which is exactly the problem in premature babies whose type II cells are not yet producing it.

How air is moved

Lungs contain no muscle of their own. They are inflated by changing the pressure around them, which follows one rule: at a fixed amount of gas, increasing the volume of a container lowers the pressure inside it, and air flows from higher to lower pressure.

In quiet inhalation the diaphragm contracts and flattens downward, and the external intercostals lift the ribs up and out. Thoracic volume increases, pressure inside the lungs falls below atmospheric, and air flows in. Quiet exhalation needs no muscle at all: the diaphragm and intercostals relax, the elastic lung recoils, volume falls, pressure rises above atmospheric, air leaves. Forced breathing recruits more: neck and chest accessory muscles for a deeper breath in, and the abdominal muscles and internal intercostals to push air out hard, which is what you use to blow out a candle or to cough.

The lungs follow the chest wall because of the pleural space between them. That space contains a thin film of fluid and sits at about negative 4 millimetres of mercury relative to atmosphere throughout the cycle, so the lung surface is effectively held against the inside of the chest wall. Puncture the chest and let air in and the seal is lost: the lung recoils away from the wall and collapses, which is a pneumothorax.

Why gases move where they do

Each gas in a mixture exerts its own share of the total pressure, called its partial pressure, and each gas diffuses independently down its own partial pressure gradient. Everything about gas exchange is that sentence applied twice.

Read the first two rows together. Oxygen is at 104 in the alveolus and 40 in the arriving blood, a gradient of 64 millimetres of mercury pushing it into the blood. Carbon dioxide is at 45 in the blood and 40 in the alveolus, a gradient of only 5 pushing it out. Carbon dioxide still leaves easily despite that tiny gradient because it is about twenty times more soluble in the fluid it must cross. At the tissues both gradients reverse, and exchange runs the other way for the same reason.

Why this matters: nothing pumps gases. Diffusion does all of it, which is why the exchange surface must be enormous and the barrier between air and blood must be thin enough to be measured in fractions of a micrometre.

Carrying the cargo

Oxygen barely dissolves in plasma: only about 1.5 percent travels that way, and the other 98.5 percent rides on haemoglobin. Carbon dioxide is handled quite differently, in three parts.

• About 7 to 10 percent stays dissolved in plasma.
• About 20 percent binds to the protein part of haemoglobin as carbaminohaemoglobin, at a different site from the oxygen.
• About 70 percent is converted to bicarbonate. Inside the red cell, carbonic anhydrase combines carbon dioxide and water into carbonic acid, which splits into hydrogen ions and bicarbonate, HCO₃^-. The bicarbonate leaves the cell into plasma in exchange for chloride coming in, which keeps charge balanced and is called the chloride shift. At the lung the whole sequence runs backwards.

This is also why breathing controls blood pH. Carbon dioxide dissolved in blood is, in effect, acid, so holding your breath acidifies the blood and over-breathing alkalises it. And the relationship works in the useful direction: a lower, more acidic pH makes haemoglobin release oxygen more readily, the Bohr effect. A hard-working muscle is producing acid and carbon dioxide, and that local acidity makes the blood passing through it hand over more oxygen without any signal being sent anywhere.

Who decides how fast you breathe

Groups of neurons in the medulla and pons set the rhythm, firing to the diaphragm through the phrenic nerves. What adjusts the rate is mostly not oxygen. Central chemoreceptors in the medulla sense the pH of the cerebrospinal fluid, which tracks blood carbon dioxide almost immediately because carbon dioxide crosses the blood brain barrier freely. Rising carbon dioxide means falling pH there, and breathing is driven harder. Peripheral chemoreceptors in the carotid and aortic bodies do respond to low oxygen, but only once it falls a long way, so they matter at altitude and in serious lung disease rather than in everyday life.

Test it on yourself. Hold your breath and the urge to breathe that builds is carbon dioxide accumulating, not oxygen running out; blood oxygen saturation is still high when the urge becomes unbearable. Breathe rapidly for thirty seconds first and you can hold longer, because you have blown carbon dioxide off below its normal level and the alarm takes longer to trigger. That is also why doing it before swimming underwater is dangerous: the alarm may not sound before oxygen genuinely runs low.

Asthma: the airway that will not let go

Asthma is a chronic inflammatory disease of the airways in which the bronchioles narrow reversibly. Three things narrow them at once. Smooth muscle in the bronchiolar wall contracts, the lining swells with inflammatory oedema, and glands produce thick mucus that plugs the smallest tubes. Over years, repeated inflammation can thicken the basement membrane and lay down collagen, a process called remodelling that makes some of the narrowing permanent.

In an allergic attack the sequence is specific. Earlier exposure produced IgE antibodies which now sit on the surface of mast cells in the airway wall. Re-exposure cross-links those antibodies, the mast cells degranulate within minutes, and histamine and leukotrienes pour out, contracting smooth muscle immediately. Hours later a second wave of inflammatory cells arrives and keeps the airway swollen, which is why an attack can worsen again after apparently settling.

The counter-intuitive part is which direction fails. Breathing in expands the chest and pulls the small airways slightly wider. Breathing out compresses them. So a narrowed bronchiole obstructs exhalation more than inhalation, air is trapped behind the narrowing, and the patient is holding too much air rather than too little. That is why the audible wheeze is on expiration and why the chest looks over-inflated in an attack.

Spirometry demonstrates it in numbers. The patient blows out as hard and long as possible. FVC is the total volume exhaled; FEV₁ is the volume exhaled in the first second. In an obstructive disease FEV₁ falls more than FVC, so their ratio drops. The test is then repeated after a bronchodilator: an increase in FEV₁ of 12 percent or 200 millilitres counts as significant reversibility, which is the feature that separates asthma from fixed obstruction.

Treatment follows the two mechanisms. A reliever, typically the short-acting beta-2 agonist salbutamol, known as albuterol in the United States, relaxes airway smooth muscle within minutes and does nothing about the inflammation. A preventer, an inhaled corticosteroid, suppresses the inflammation over weeks and does nothing immediately. Someone relying on a reliever several times a week is treating a symptom whose cause is untreated, and current guidance increasingly pairs an inhaled corticosteroid with the reliever so that every rescue dose also treats inflammation.

Common misconceptions

• The urge to breathe comes from low oxygen. It comes from rising carbon dioxide detected as falling pH in the brain. Oxygen only takes over as the driver when it has fallen a long way.
• In an asthma attack the patient cannot get air in. The obstruction is worse on the way out. Air is trapped, the chest is over-inflated, and the wheeze is heard on expiration.
• A reliever inhaler treats asthma. It relaxes muscle for a few hours. The inflammation underneath is treated by the steroid preventer, which is why frequent reliever use is a warning sign rather than good control.

The short version

Air passes twenty-odd branchings from trachea to alveolus, losing cartilage and gaining smooth muscle on the way, and only the last few generations exchange any gas. Alveolar walls are one flattened cell thick with type II cells making the surfactant that stops them collapsing. Breathing in is active, using diaphragm and external intercostals to enlarge the chest and lower the pressure inside it; quiet breathing out is elastic recoil. Gases move only by diffusion down partial pressure gradients: oxygen from 104 to 40 millimetres of mercury at the alveolus, carbon dioxide across a gradient of just 5 that works because it is far more soluble. Oxygen travels on haemoglobin, carbon dioxide mostly as bicarbonate made by carbonic anhydrase, and acid conditions in a working tissue make haemoglobin let go of more oxygen. Carbon dioxide, not oxygen, sets the breathing rate. In short: asthma is narrowing of the one part of the airway built to change diameter, it obstructs exhalation more than inhalation, and its two treatments act on two different parts of the problem.

Sources

1. OpenStax. (2022). 22.3 The process of breathing. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 22.5 Transport of gases. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Goldin, J., and Cataletto, M. E. (2024). Asthma. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. National Heart, Lung, and Blood Institute. (2024). Asthma. National Institutes of Health. NHLBI

Chew bread for a minute

Put a piece of plain white bread in your mouth and chew it without swallowing for a full sixty seconds. It turns sweet. Nothing sweet was added. What happened is that an enzyme in your saliva, salivary amylase, has been cutting the long starch chains in the bread into short sugars that your taste receptors can detect. Digestion has already begun, before the food has moved a centimetre.

That is the whole principle. Food is made of polymers far too large to cross a cell membrane, and the digestive tract is a nine-metre chemical processing line whose job is to cut them into monomers small enough to absorb, and then to absorb them. This lesson follows one sandwich the length of that line.

The line, and the wall it is built from

The alimentary canal runs mouth, pharynx, oesophagus, stomach, small intestine in three parts, large intestine, rectum, anus. Four accessory organs sit outside the tube and empty into it: the salivary glands, the liver, the gallbladder and the pancreas. Food never enters them.

From oesophagus to anus the wall has the same four layers, and knowing them makes every digestive disease easier to place. The mucosa lines the lumen and does the secreting and absorbing. The submucosa beneath it carries blood vessels, lymphatics and nerves. The muscularis is two sheets of smooth muscle, one circular and one longitudinal, and it does the moving. The serosa is the slippery outer wrap, continuous with the peritoneum from lesson one.

Those two muscle sheets produce two motions. Peristalsis is a wave of circular contraction behind the food and relaxation ahead of it, squeezing the contents forward, and it works against gravity: an astronaut, or a person standing on their head, still swallows successfully. Segmentation is local contraction in alternating patches that chops and mixes without moving anything along, and it dominates in the small intestine, where the priority is contact with the wall rather than speed.

Mouth and oesophagus

Teeth do mechanical digestion, cutting and grinding to increase surface area for enzymes, and the tongue forms the result into a bolus. Saliva supplies water, mucus, lysozyme and salivary amylase, which begins on starch and is then destroyed by stomach acid, so its window is short.

Swallowing is voluntary at the start and reflex after that. The soft palate rises to block the nasal cavity and the epiglottis folds over the larynx, and the bolus passes into the oesophagus, which delivers it to the stomach in a few seconds.

The stomach: acid, and a protein enzyme that needs it

The stomach is a muscular bag with a third, oblique layer of muscle for churning. Its lining contains gastric pits with several cell types. Parietal cells secrete hydrochloric acid, holding stomach contents between about pH 1.5 and 3.5, and also intrinsic factor, without which vitamin B₁₂ cannot be absorbed further down. Chief cells secrete pepsinogen, an inactive precursor. Mucous cells coat everything with an alkaline mucus layer.

The acid does three jobs: it kills most swallowed microorganisms, it unfolds proteins so enzymes can reach their bonds, and it converts pepsinogen into active pepsin. Secreting the enzyme in an inactive form is not an accident of chemistry but a safety measure: pepsin digests protein, and the cell that makes it is made of protein. The same logic explains the alkaline mucus. When that barrier fails, from infection with Helicobacter pylori or from drugs that suppress mucus production, the stomach digests its own wall, which is a peptic ulcer.

The result is chyme, an acidic soup released in small squirts through the pyloric sphincter over several hours. Almost nothing is absorbed here; the stomach is a tank and a protein-digestion chamber, not an absorptive organ.

The duodenum, where everything arrives at once

The first 25 centimetres of small intestine receive three things at the same moment: acidic chyme from the stomach, pancreatic juice, and bile.

Pancreatic juice is alkaline with bicarbonate, which neutralises the acid so the pancreatic enzymes can work at all, and it carries the main enzyme set. Bile is made continuously by the liver, stored and concentrated in the gallbladder, and released when fat arrives. Bile contains no enzymes. Bile salts are detergents: one end dissolves in fat, the other in water, so they break large fat droplets into tiny ones, which is emulsification. That does not digest anything; it multiplies the surface area on which lipase can act, and lipase can only work at a surface.

The last four are not released into the lumen. They are anchored in the membrane of the absorbing cells themselves, so the final cut happens at the doorway. Lactase is the one that most often disappears: in most of the world's adults its production declines after childhood, and undigested lactose then passes to the colon where bacteria ferment it, producing the gas, bloating and diarrhoea of lactose intolerance. That is an enzyme shortage, not an immune reaction, and it is the cleanest contrast with coeliac disease later in this lesson.

Absorption, and why the surface is folded three times over

The small intestine has to absorb nearly everything, so its surface is amplified at three scales. The whole lining is thrown into circular folds. Each fold is covered in villi, finger-like projections about a millimetre long. Each cell on a villus carries thousands of microvilli, forming the brush border. The result is an absorptive area vastly larger than the tube's own dimensions would allow.

Inside each villus is a capillary network and one blind-ended lymphatic vessel, a lacteal. Sugars and amino acids cross into the capillaries. Fats take the other route: fatty acids and monoglycerides are ferried to the cell surface inside micelles formed by bile salts, are absorbed, are reassembled into triglycerides inside the cell, packaged with protein, and released into the lacteal, because the packages are too large for a capillary. They reach the blood only later, at the thoracic duct.

Blood leaving the intestine does not go straight to the heart. It collects into the hepatic portal vein and goes to the liver first. The core of it: everything absorbed from the gut except fat is inspected and processed by the liver before the rest of the body sees it, which is why the liver stores glucose as glycogen after a meal and why many drugs lose much of their dose on the way through.

The large intestine

By the time material reaches the colon, nutrient absorption is essentially finished. The colon reclaims water and electrolytes, turning liquid residue into formed faeces over twelve to twenty-four hours. It also houses the gut microbiota, which ferment fibre the human enzymes cannot touch, produce short-chain fatty acids the colon cells use for fuel, and synthesise some vitamin K and B vitamins. Move material through too fast and water is not reclaimed, which is diarrhoea; too slow and too much is, which is constipation.

Reading a label as chemistry

A nutrition label is a description of what is about to enter that tract. Four things make it readable.

Serving size comes first and everything else is per serving. It reflects the amount people typically eat, not a recommendation, so a bottle holding two servings delivers double every number printed.

Calories come from three macronutrients at fixed rates: carbohydrate and protein at about 4 kilocalories per gram, fat at about 9. Check any label with that arithmetic and it comes out close. Fat is the densest store because it is the least oxidised of the three and carries no water with it, which is also why the body stores surplus energy as fat rather than as glycogen.

Percent Daily Value uses 2000 calories a day as a general guide. The practical rule is that 5 percent DV or less per serving is low in that nutrient and 20 percent or more is high. The FDA identifies saturated fat, sodium and added sugars as the ones to get less of, and dietary fibre, vitamin D, calcium, iron and potassium as the ones to get more of.

Total carbohydrate covers several different things. Fibre is carbohydrate your enzymes cannot digest, so it reaches the colon intact and is not absorbed as sugar; added sugars are carbohydrate already cut into monomers, needing no digestion at all and appearing in the blood fast. Both are counted inside total carbohydrate. That single distinction explains why two foods with identical total carbohydrate can behave completely differently once they reach the small intestine.

Coeliac disease: losing the villi

Coeliac disease affects roughly 1 percent of people worldwide. Gluten from wheat, barley and rye is broken into peptides including gliadin. An enzyme called tissue transglutaminase modifies those peptides, which then bind tightly to particular immune molecules, HLA-DQ2 or HLA-DQ8, carried by nearly everyone who develops the disease. That binding activates T cells, and the resulting immune attack falls on the lining of the small intestine.

The damage is exactly what you would predict from the anatomy. Villi flatten, which is villous atrophy, and the crypts between them deepen. Surface area collapses, and with it the capacity to absorb, so the illness presents as diarrhoea, weight loss, anaemia from failed iron absorption, and in children as poor growth. In a healthy intestine the ratio of villus height to crypt depth is around 3 to 1; in untreated coeliac disease it is often below 2 to 1.

Diagnosis begins with a blood test for antibodies against tissue transglutaminase, tTG-IgA, which has a high negative predictive value, so a negative result in a low-risk person effectively rules the condition out. Most patients then need a small intestinal biopsy to confirm it and gauge the damage, and both tests must be done while the patient is still eating gluten, since the lining begins repairing itself once gluten stops. The only effective treatment is complete, lifelong exclusion of gluten, on which most patients improve within weeks and the intestinal lining typically heals over six to twenty-four months.

Common misconceptions

• Bile digests fat. Bile contains no enzymes. It emulsifies, breaking large droplets into small ones so that pancreatic lipase, which works only at a surface, has far more surface to work on.
• Coeliac disease and lactose intolerance are the same kind of problem. Lactose intolerance is a missing brush border enzyme. Coeliac disease is an immune attack that destroys the absorptive surface itself.
• Food is absorbed all along the tract. Almost all absorption happens in the small intestine. The stomach is a holding and protein-digestion chamber, and the colon mainly reclaims water.

What to carry forward

The digestive tract is a tube of four layers moved by peristalsis and mixed by segmentation, with four accessory organs secreting into it. Salivary amylase starts on starch; stomach acid unfolds protein and activates pepsin from its safe inactive precursor; the duodenum receives alkaline pancreatic juice with amylase, proteases and lipase, plus bile salts that emulsify fat without digesting it. Brush border enzymes make the final cut at the membrane of the absorbing cell, and folds, villi and microvilli multiply that membrane enormously. Sugars and amino acids leave in the hepatic portal vein and are processed by the liver before reaching the body; fats leave through lacteals into lymph. A nutrition label is per serving, runs on 4, 4 and 9 kilocalories per gram, and uses a 2000 calorie reference with 5 percent low and 20 percent high. Remember: coeliac disease destroys the villi, and every one of its symptoms follows from the lost surface area.

Sources

1. OpenStax. (2022). 23.7 Chemical digestion and absorption: a closer look. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 23.1 Overview of the digestive system. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Daley, S. F., and Haseeb, M. (2025). Celiac disease. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. U.S. Food and Drug Administration. (2024). How to understand and use the nutrition facts label. FDA

A hundred and eighty litres in, one and a half out

Your two kidneys filter plasma at about 125 millilitres a minute in an adult man and about 105 in an adult woman. Run that for a day and it comes to roughly 180 litres of filtrate, against a total blood volume of five. Yet you pass one to two litres of urine. Ninety-nine percent of everything filtered is taken back.

That looks like an absurd way to build an organ. Why push out 180 litres in order to keep 178? The answer is that filtering indiscriminately and then choosing carefully what to reclaim is far more controllable than trying to pick individual waste molecules out of blood. The kidney throws almost everything away and then buys back exactly what the body needs today. This lesson follows one volume of filtrate through that process, segment by segment.

The organ and the unit

A kidney is about the size of a fist, sitting against the back wall of the abdomen behind the peritoneum. Cut one lengthwise and there are two zones: an outer cortex and an inner medulla arranged as triangular pyramids whose tips point inward. Urine dripping from the pyramid tips collects in the renal pelvis, leaves through the ureter, is stored in the bladder, and exits through the urethra.

The working unit is the nephron, and each kidney holds roughly a million of them. One nephron has two parts: a filter and a tube. The filter is the renal corpuscle, a knot of capillaries called the glomerulus sitting inside a cup called Bowman's capsule. The tube runs: proximal convoluted tubule in the cortex, then the loop of Henle diving down into the medulla and back, then the distal convoluted tubule in the cortex, then a collecting duct that runs back down through the medulla to the pyramid tip.

The blood supply contains one oddity worth pausing on. Blood enters the glomerulus through an afferent arteriole and leaves through an efferent arteriole. A capillary bed sandwiched between two arterioles is found almost nowhere else in the body, and it exists so that pressure inside the glomerulus can be set independently of the rest of the circulation: constrict the outflow arteriole and glomerular pressure rises even if blood pressure has not. Blood then passes into a second capillary network, the peritubular capillaries, wrapping the tubule, which is where everything reabsorbed goes.

Step one: filtration

Filtration is pressure-driven and unselective within a size limit. Three barriers lie between blood and capsule: fenestrated capillary endothelium punched with pores, a basement membrane, and a layer of cells called podocytes whose interlocking foot processes leave narrow filtration slits. Water, ions, glucose, amino acids and urea pass. Blood cells and plasma proteins do not, partly by size and partly because the membrane carries a negative charge that repels albumin.

The arithmetic is the capillary tug of war from lesson eleven, with different numbers. Glomerular blood pressure pushes out at about 55 millimetres of mercury. Against it, the pressure of fluid already in the capsule pushes back at about 15, and the colloid osmotic pull of plasma proteins pulls in at about 30. Net filtration pressure is 55 minus 45, about 10 millimetres of mercury outward. A small number, applied across a million nephrons, gives 180 litres a day.

What matters here: finding protein or blood cells in urine means the filter itself is damaged, because those should never have crossed. Finding glucose means something different, and the next section explains why.

Step two: the proximal convoluted tubule reclaims the bulk

About 67 percent of the water, sodium and potassium entering the nephron is reabsorbed here, along with essentially all of the glucose and amino acids. The cells lining it have a dense brush border of microvilli and are packed with mitochondria, because most of this is active transport.

The engine is the sodium-potassium pump on the side of the cell facing the blood. It keeps sodium low inside the cell, so sodium flows in from the tubule down its gradient, and the cell exploits that flow by coupling it to other molecules. Glucose rides in alongside sodium on a co-transporter. Water follows by osmosis.

Because those transporters are proteins, there is a finite number of them, and once every one is occupied the excess cannot be reclaimed. Reabsorption of glucose is maximal at about 375 milligrams a minute in men and 300 in women, which corresponds to a blood concentration around 200 milligrams per decilitre. Below that, urine contains no glucose at all. Above it, glucose spills into the urine, and because glucose holds water osmotically, the patient passes large volumes and is thirsty. That is the original meaning of the name: diabetes from the Greek for a siphon, mellitus from the Latin for honeyed. The sweet urine of untreated diabetes is a transporter being overwhelmed, not a kidney disease.

Step three: the loop of Henle builds a salt gradient

Filtrate leaving the proximal tubule is still about the same concentration as blood plasma, near 300 milliosmoles per kilogram. To make concentrated urine the kidney needs somewhere concentrated to draw water into, and the loop of Henle builds it.

The two limbs have opposite properties, which is the entire trick. The descending limb is permeable to water through permanent aquaporin channels but does not transport salt. The ascending limb is completely impermeable to water but actively pumps sodium and chloride out into the surrounding tissue.

Follow the consequences. The ascending limb pumps salt into the medulla, making the medullary tissue salty. Fluid descending beside it, in the opposite direction, loses water by osmosis into that salty tissue and so becomes more concentrated as it goes down. That more concentrated fluid then rounds the bend and enters the ascending limb, where there is now more salt available to pump out. The two limbs feed each other, and because they run in opposite directions the small difference achievable at any single level is multiplied along the length of the loop. The result is a standing gradient reaching about 1200 milliosmoles per kilogram at the tip of the medulla, four times the concentration of blood.

Fluid entering the distal tubule is actually more dilute than plasma, because the ascending limb removed salt without water. The concentrated medulla is not for the loop's benefit. It is there for the collecting duct.

Step four: the fine adjustment

The distal convoluted tubule and the collecting duct handle a small fraction of the filtrate and decide almost everything about the final urine, because this is the part under hormonal control.

Antidiuretic hormone, released from the posterior pituitary when blood becomes too concentrated, inserts aquaporin water channels into the collecting duct wall. The duct runs down through that 1200 milliosmole medulla, so with the channels in place water is pulled out of it all the way down, producing a small volume of concentrated urine. With no antidiuretic hormone, the duct stays waterproof and dilute urine passes straight through. Alcohol suppresses the hormone, which is why drinking it produces more urine than the volume drunk.

Aldosterone, from the adrenal cortex, increases sodium reabsorption here and potassium secretion in exchange. Water follows the sodium, so the effect is to conserve fluid and raise blood pressure, which is the slow arm of blood pressure control from lesson eleven.

Secretion also happens here and along the tubule: hydrogen ions, potassium, ammonium, and many drugs are moved from blood into filtrate rather than being filtered. This is how the kidney fine-tunes blood pH, and it is the second reason your blood pH holds between 7.35 and 7.45, the first being the carbon dioxide system in the lungs. Lungs adjust within minutes; kidneys within hours to days, and more completely.

When nephrons are lost

Chronic kidney disease is the gradual, usually irreversible loss of functioning nephrons, most often from long-standing diabetes or high blood pressure, both of which damage the delicate glomerular capillaries. It is staged by estimated glomerular filtration rate in millilitres per minute per 1.73 square metres of body surface.

The disease is usually silent until late, because surviving nephrons enlarge and work harder, so a patient can lose well over half their function and feel nothing. What eventually appears follows directly from the jobs the kidney was doing: nitrogenous waste builds up, fluid is retained and blood pressure rises, potassium climbs and threatens the heart, acid accumulates, and the kidney's endocrine work fails too. It stops making erythropoietin, so anaemia develops, and it stops performing the final activation of vitamin D, so calcium absorption falls and bone suffers, which is the chain from lessons three and four failing at its last link.

What a dialysis machine is doing

Haemodialysis replaces filtration and nothing else. Blood is led out of the body through a surgically created connection between an artery and a vein, usually in the forearm, which over weeks makes a vein thick enough to be needled repeatedly. In the machine, blood runs through bundles of hollow fibres whose walls are semipermeable, and a fluid called dialysate flows around the outside in the opposite direction. Wastes and excess ions diffuse from blood into dialysate down their concentration gradients; the dialysate composition is chosen so that things the patient needs, such as calcium and bicarbonate, do not leave, and things to be removed have no counterpart on the far side. Pressure across the membrane pulls off surplus water. A typical in-centre schedule is three sessions a week of about four hours each.

Notice what dialysis does not do. It does not make erythropoietin, so those patients usually need that hormone given as a drug. It does not activate vitamin D. It does not adjust minute by minute, so wastes and fluid accumulate between sessions and are cleared in bursts. That is why a transplant, which restores every function at once including the endocrine ones, remains the better outcome where it is possible.

Common misconceptions

• The kidney filters out the waste. It filters out nearly everything small, waste and useful alike, and then reclaims 99 percent of the volume. Selectivity lives in reabsorption, not in filtration.
• Glucose in urine means the kidney is damaged. It usually means blood glucose has exceeded what the transporters can carry back, which points at diabetes rather than at the kidney.
• The loop of Henle concentrates urine. Fluid leaving the loop is more dilute than plasma. The loop builds the salty medulla; the collecting duct uses it, and only when antidiuretic hormone says so.

Putting it together

Each kidney holds about a million nephrons, and each nephron filters at a glomerulus sandwiched between two arterioles so its pressure can be set independently. Net filtration pressure of about 10 millimetres of mercury produces 180 litres of filtrate a day, of which 99 percent returns. The proximal tubule reclaims about two thirds of water and salt and all the glucose, up to a transport maximum that untreated diabetes exceeds. The loop of Henle, permeable to water going down and to salt coming up, multiplies a small difference into a medulla of 1200 milliosmoles per kilogram. The collecting duct then passes through that medulla and loses as much water as antidiuretic hormone allows, while aldosterone sets sodium retention and secretion handles pH. The upshot: chronic kidney disease removes nephrons one at a time and stays silent until most are gone, and dialysis replaces the filtering but not the hormones, which is why transplantation remains the better repair.

Sources

1. OpenStax. (2022). 25.5 Physiology of urine formation. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 25.6 Tubular reabsorption. In Anatomy and Physiology 2e. Rice University. openstax.org
3. National Institute of Diabetes and Digestive and Kidney Diseases. (2018). Hemodialysis. National Institutes of Health. NIDDK
4. National Institute of Diabetes and Digestive and Kidney Diseases. (2024). Chronic kidney disease. National Institutes of Health. NIDDK

Twelve to eighteen

Measles has a basic reproduction number of about 12 to 18: one case introduced into an unprotected population generates, on average, that many further cases. Almost nothing else transmits that efficiently. The proportion of a population that must be immune to stop sustained spread is one minus one over that number, which for measles works out at roughly 92 to 94 percent. Seasonal influenza, with a reproduction number near 1.3, needs about 23 percent.

Those numbers are arithmetic, not biology. But the immunity they count is entirely biological, and it has two completely different sources: a response you are born with that is fast and general, and one you build over your lifetime that is slow, specific, and remembers.

The first line is not immune at all

Before either system engages there are barriers, and most infections never get past them. Intact skin with its dead keratinised layer is one, already met in lesson three. Mucous membranes trap organisms in sticky mucus and sweep them away on cilia. Stomach acid at pH 1.5 to 3.5 kills most swallowed organisms. Tears and saliva carry lysozyme, which digests bacterial cell walls. Urine flushes the urethra. Beneficial resident bacteria occupy surfaces and compete with newcomers for space and nutrients. These are physical and chemical, not immune responses, and they need no recognition of anything.

Innate immunity: fast, general, no memory

If something crosses a barrier, the innate system responds within minutes, the same way every time, to broad classes of molecule that microbes have and human cells do not.

Inflammation is the coordinated local result, and its four classic signs follow from the mechanism rather than being a list to memorise. Damaged tissue and mast cells release histamine and other mediators. Local arterioles dilate, which brings more blood: hence redness and heat. Capillaries become more permeable, so fluid and proteins leak into the tissue: hence swelling. The swelling and the mediators stimulate pain receptors: hence pain. The leaked fluid delivers complement and antibodies, and the wider vessels let neutrophils squeeze through the wall to the site. Fever, from lesson one, is the systemic version: pyrogens raise the hypothalamic set point, and the higher temperature slows some pathogens while speeding immune enzymes.

Adaptive immunity: slow, specific, remembers

The adaptive system recognises one particular molecular shape, an antigen, and does so through receptors generated by shuffling gene segments during lymphocyte development. The result is a population of lymphocytes among which almost any shape has some cell able to bind it, before that shape has ever been encountered.

That raises the obvious problem: a randomly generated receptor could just as easily match one of your own molecules. The solution is deletion. During development, lymphocytes that bind the body's own tissues strongly are destroyed or disabled, mostly in the thymus for T cells and the bone marrow for B cells. That process is called tolerance, and its failure is autoimmunity, which this lesson ends on.

Two branches share the work. B cells handle the humoral response: when activated they divide into plasma cells that secrete antibodies, soluble versions of their own receptor, which act on anything outside cells. T cells handle the cell-mediated response and act on cells, which is what you need against a virus already hiding inside one.

T cells cannot see free antigen. They only recognise fragments displayed on the surface of a cell in a groove formed by a major histocompatibility complex molecule, called HLA in humans. Every nucleated cell displays samples of what it is making internally, so a virus-infected cell advertises the fact. Macrophages and dendritic cells additionally display fragments of what they have eaten.

The helper T cell's central position explains HIV. The virus infects CD4 cells specifically, and as their numbers fall the coordinator of both branches disappears, so the patient becomes vulnerable to organisms a healthy immune system handles without difficulty.

Clonal selection, and why the second time is different

When a lymphocyte with the right receptor finally meets its antigen and receives the necessary helper signals, it divides repeatedly, producing a clone of identical cells all specific for that antigen. Most become effector cells that fight now. Some become memory cells that do nothing at all and persist, sometimes for decades.

Plot antibody concentration against time and the two exposures look completely different. The primary response has a lag of about one to two weeks while the rare matching cell is found and the clone is built, reaches a modest peak, and is dominated by IgM. The secondary response starts within two or three days, because thousands of memory cells already exist, climbs perhaps a hundredfold higher, consists mainly of IgG, and lasts much longer. On a graph it is a small late hill followed by a tall early mountain. The point: you usually do not fall ill the second time, not because the pathogen is weaker but because the lag has gone.

An antibody is Y-shaped, built from two heavy and two light protein chains. The tips of the two arms are the variable regions, and their shape is what binds antigen. The stem is constant within a class and determines what happens next. IgG is the main circulating class and the only one crossing the placenta, so a newborn carries its mother's antibodies for months. IgM is made first and is efficient at clumping. IgA guards mucous surfaces and appears in breast milk. IgE is the one bound to mast cells, and cross-linking it is what triggers the asthma attack and the allergic reaction from lesson twelve.

Antibodies do four things: neutralise, by covering the part of a pathogen that would attach to a cell; opsonise, by coating a microbe so phagocytes grip it; agglutinate, by clumping many microbes with their two arms; and activate complement.

Vaccines, and what herd immunity actually is

A vaccine supplies the antigen without the disease, so that the primary response and its memory cells are generated in advance. When the real pathogen arrives it meets a secondary response.

Herd immunity is the population-level consequence. If enough people around an infected person are immune, most of their contacts are dead ends and a chain of transmission stops. The threshold is one minus one over the reproduction number, which is why the figure differs so much between diseases: about 23 percent for seasonal influenza, roughly 92 to 94 percent for measles. That is also why measles returns first when coverage slips: it has the least slack of almost any vaccine-preventable disease.

When tolerance fails

Autoimmunity is an adaptive response mounted against the body's own molecules, and three conditions from earlier lessons are examples. Multiple sclerosis attacks central nervous system myelin. Coeliac disease involves an immune response to modified wheat peptides that damages the intestinal lining. And type 1 diabetes destroys the beta cells of the pancreatic islets.

Take the third in detail, since lesson nine left it unexplained. In type 1 diabetes, cytotoxic T cells destroy insulin-producing beta cells while the rest of the pancreas, including the glucagon-producing alpha cells next door, is left intact. The process runs silently over months to years before enough beta cells are gone for blood glucose to rise. Autoantibodies against several islet targets can be detected in blood, among them antibodies to insulin itself, to an enzyme called glutamic acid decarboxylase 65, to islet antigen 2, and to a zinc transporter. They are markers of the attack rather than its main weapon, and they can appear years before symptoms.

Susceptibility is strongly tied to the same HLA molecules that present antigen to T cells: HLA DR4-DQ8 and DR3-DQ2 are present in about 90 percent of children with the condition, and HLA genes account for roughly 40 percent of the familial clustering. Notice that DQ2 and DQ8 are the same molecules implicated in coeliac disease, which is one reason the two conditions turn up together more often than chance would predict. Because the destroyed cells do not come back, treatment is lifelong replacement of the missing hormone rather than suppression of the immune system, which arrives too late to matter once the beta cells are gone.

Common misconceptions

• Vaccines give you a weak version of the illness. Only live attenuated vaccines contain a replicating organism, and it is altered so it cannot cause the disease. Subunit, toxoid and mRNA vaccines contain no organism capable of replicating at all.
• Herd immunity is the same percentage for every disease. It is one minus one over the reproduction number, so it ranges from roughly a quarter of the population for seasonal influenza to well over ninety percent for measles.
• Antibodies kill pathogens. They bind them. Killing is done by phagocytes that grip the coated microbe, by complement that punches holes, and by cytotoxic T cells for infected body cells.

What you now know

Barriers stop most infections without recognising anything. Innate immunity responds in minutes, identically every time, through phagocytes, natural killer cells, complement, interferons and the inflammatory response whose four signs follow directly from vasodilation and increased permeability. Adaptive immunity recognises a specific antigen using receptors generated at random, with self-reactive lymphocytes deleted during development. B cells make antibodies against anything outside a cell; T cells recognise fragments presented on HLA molecules, with helper cells coordinating and cytotoxic cells killing infected cells. Clonal selection produces effectors now and memory cells for later, which is why the secondary response is faster, larger and longer. Vaccines create that memory in advance, and the herd immunity threshold is one minus one over the reproduction number. Worth holding on to: type 1 diabetes is that entire apparatus aimed at one cell type in one organ, which is why it produces an endocrine disease rather than an infectious one.

Sources

1. OpenStax. (2022). 21.2 Barrier defenses and the innate immune response. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 21.4 The adaptive immune response: B lymphocytes and antibodies. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Lucier, J., and Mathias, P. M. (2024). Type 1 diabetes. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. Centers for Disease Control and Prevention. (2024). Epidemiology and prevention of vaccine-preventable diseases (14th ed.). Public Health Foundation.

Every egg was there before she was born

A female infant is born with one to two million primary oocytes in her ovaries. By puberty the number has fallen to roughly 400,000, and by the end of menopause it reaches zero. No new ones are made. Each of those cells began meiosis during fetal development and then stopped, part way through the first division, and waits there for years or decades until a cycle selects it.

Male gamete production is the opposite in almost every respect: nothing is stockpiled, production begins at puberty and continues, and each sperm takes a little over two months to make from start to finish. Two strategies, one purpose. This lesson covers the anatomy of both systems, the two interlocking cycles that run in the female body every month, and what happens if a sperm and an egg meet.

The male reproductive system

The testes sit outside the body cavity in the scrotum, and the reason is temperature: sperm production works best a couple of degrees below core body temperature. Inside each testis are tightly coiled seminiferous tubules, where sperm are made. Two other cell types live there. Sertoli cells line the tubules, nurse the developing sperm and form a barrier protecting them. Leydig cells sit between the tubules and secrete testosterone.

Sperm leaving the tubules are not yet able to swim. They spend time maturing in the epididymis, a heavily coiled tube on the back of each testis, and are then carried through the vas deferens into the pelvis. Three glands add fluid. The seminal vesicles contribute the largest share, rich in fructose which is the sperm's fuel. The prostate adds a milky alkaline fluid that helps neutralise vaginal acidity. The bulbourethral glands add a small amount of lubricating mucus. Sperm plus these secretions is semen, and it leaves through the urethra, which in the male carries both urine and semen but never at the same time.

A mature sperm has three parts and no spare material at all. The head carries the nucleus with a single set of 23 chromosomes, capped by the acrosome, a vesicle of enzymes for digesting a path through the coverings of an egg. The midpiece is packed with mitochondria supplying ATP. The tail is a flagellum. Production takes a little over two months per cell, and the controlling hormones come from the anterior pituitary: luteinising hormone drives the Leydig cells to make testosterone, and follicle stimulating hormone acts on the Sertoli cells to support sperm production.

The female reproductive system

The ovaries sit in the pelvis and do two jobs: they house and mature the oocytes, and they secrete oestrogen and progesterone. Beside each ovary opens a uterine tube, also called a fallopian tube, whose fringed end sweeps the released oocyte inward and whose cilia move it along. Fertilisation, if it happens, happens here rather than in the uterus.

The uterus is a thick-walled muscular organ with two layers that matter. The myometrium is smooth muscle and does the work of labour. The endometrium is the lining, and it is rebuilt and shed on a cycle. The lower narrow end is the cervix, which opens into the vagina. The external structures, collectively the vulva, include the labia majora and minora and the clitoris, which is dense in sensory nerve endings and is the female homologue of the penis, developing from the same embryonic tissue.

Two cycles, running together

What is usually called the menstrual cycle is two cycles at once: one in the ovary and one in the uterus, coordinated by the same hormones. The average length is about 28 days, and cycles from roughly 21 to 35 days are within the normal range. Day 1 is the first day of bleeding.

Follow the hormones and the whole thing becomes one feedback loop with a twist. The hypothalamus releases gonadotropin releasing hormone, which makes the anterior pituitary release follicle stimulating hormone and luteinising hormone. Follicle stimulating hormone drives follicles to grow, and growing follicles secrete oestrogen. Oestrogen at low and moderate levels feeds back negatively, restraining the pituitary, which is the ordinary pattern from lesson nine.

Then it inverts. When oestrogen from the dominant follicle stays high for a couple of days, the feedback flips from negative to positive: high oestrogen now stimulates rather than suppresses, and the pituitary releases a sharp surge of luteinising hormone. That surge ruptures the follicle within about a day. This is the second clear example of positive feedback in the course, after labour and clotting in lesson one, and it has the same signature: it escalates and needs an event to end it.

After ovulation the emptied follicle becomes the corpus luteum, a temporary endocrine gland secreting progesterone. Progesterone maintains the thickened endometrium and suppresses the pituitary so no further follicle develops. The corpus luteum has a fixed working life of 10 to 12 days unless it is rescued. If no embryo implants, it degenerates, progesterone falls, the endometrium loses its support, and it is shed. That is menstruation, and the falling progesterone also releases the pituitary, so follicle stimulating hormone rises and the next cycle begins.

Why this matters: hormonal contraception works on exactly this loop. Supplying steady oestrogen and progestogen keeps the negative feedback switched on, so follicle stimulating hormone and luteinising hormone stay low, no dominant follicle matures, and the luteinising hormone surge never happens. No surge means no ovulation.

Fertilisation and the first week

An oocyte survives roughly a day after ovulation; sperm can survive several days in the female tract. Fertilisation happens in the upper part of a uterine tube. Many sperm reach the egg, and their acrosomal enzymes digest the surrounding cells and the glycoprotein coat. When one sperm membrane fuses with the oocyte membrane, the oocyte immediately changes that coat chemically so no second sperm can enter, which matters because two sperm would give three sets of chromosomes and a non-viable embryo. The oocyte then completes the meiotic division it suspended before birth, and the two nuclei combine into a zygote with 46 chromosomes, 23 from each parent.

The zygote divides as it travels down the tube, without growing, so the cells get smaller each time: 2, 4, 8, 16. By about day 4 it is a solid ball called a morula, and by day 5 it has hollowed into a blastocyst with an outer shell that will become the placenta and an inner cell mass that will become the embryo. Around days 6 to 10 it embeds in the secretory endometrium, which is implantation.

The outer shell immediately begins secreting human chorionic gonadotropin, and its job is precisely to rescue the corpus luteum from the death it was scheduled for. The corpus luteum keeps producing progesterone, the endometrium is maintained, and menstruation does not occur. That hormone appears in blood and then urine within days of implantation, and detecting it is exactly what a home pregnancy test does. Around the end of the first trimester the placenta takes over progesterone production and the corpus luteum is no longer needed.

From embryo to birth

The placenta is the exchange organ, built from both fetal and maternal tissue. Fetal blood in the villi and maternal blood in the spaces around them come within a few cell layers of each other but do not mix. Oxygen, glucose, amino acids and antibodies of the IgG class cross to the fetus; carbon dioxide and waste cross back. Some things cross that should not, including alcohol, nicotine, many drugs and some viruses, which is the reason for caution in pregnancy.

The embryonic period runs from fertilisation to the end of week 8. During it the three germ layers from lesson two form, and every organ system is laid down, a process called organogenesis. Because structures are being built rather than merely grown, this is when development is most vulnerable to damaging agents, and often before a pregnancy has been recognised. From week 9 to birth is the fetal period, dominated by growth and by maturation of organs that already exist; the lungs are among the last to become functional, which is why surfactant from lesson twelve is the limiting factor in very premature birth. Full-term pregnancy is about 40 weeks counted from the last menstrual period, which is roughly 38 weeks from conception.

Labour has three stages. In the first, uterine contractions dilate the cervix from closed to about 10 centimetres, and this is by far the longest. In the second, the baby is delivered. In the third, the placenta is delivered. The driving mechanism is the positive feedback loop from lesson one: the head presses on the cervix, stretch receptors signal the hypothalamus, oxytocin is released from the posterior pituitary, the uterus contracts harder, the head presses harder. The loop ends only when the baby is born and the stimulus is removed.

Endometriosis

Endometriosis is the presence of tissue resembling the uterine lining outside the uterus, and it affects an estimated 10 percent of women of reproductive age worldwide. The commonest site is the ovary, followed by the ligaments and folds of peritoneum around the uterus; occasionally deposits appear on bowel or bladder. The most widely accepted explanation is retrograde menstruation, in which some endometrial tissue flows backwards along the uterine tubes into the pelvis, though since retrograde flow is common and endometriosis is not, other factors must decide who develops it.

The deposits respond to the same hormones as the lining they resemble, so they thicken and bleed with each cycle. Blood released into the pelvis cannot leave, and the result is inflammation, scarring and adhesions binding organs together. The hallmark symptoms are chronic cyclic pelvic pain, severe period pain, pain during intercourse, pain on passing stool or urine, and difficulty conceiving. Severity of symptoms does not track the amount of tissue found, so a small deposit in the wrong place can cause more pain than an extensive one elsewhere.

Diagnosis is frequently delayed by years, partly because severe period pain is often treated as normal. Transvaginal ultrasound and MRI are the main imaging tools, and current guidance supports diagnosing and treating on history and imaging rather than requiring surgery first, though laparoscopy remains the definitive confirmation. First-line treatment uses combined hormonal contraceptives, progestins and anti-inflammatory painkillers, all of which work by suppressing the cyclical hormonal stimulation that makes the deposits bleed. Where that fails, GnRH agonists or antagonists shut the axis down further, and surgical excision of the deposits is used for persistent disease.

Common misconceptions

• Ovulation always happens on day 14. It happens about 14 days before the next period, so in a longer or shorter cycle the follicular phase varies while the luteal phase stays near 10 to 12 days.
• Fertilisation happens in the uterus. It happens in the upper part of a uterine tube. The embryo reaches the uterus several days later, as a blastocyst.
• Severe period pain is just something to put up with. Pain that stops normal activity is the commonest presenting symptom of endometriosis, and treating it as normal is a documented cause of years of diagnostic delay.

Looking back

Sperm are made continuously from puberty in the seminiferous tubules, matured in the epididymis, and mixed with secretions from the seminal vesicles, prostate and bulbourethral glands; Leydig cells make testosterone under luteinising hormone and Sertoli cells support production under follicle stimulating hormone. Oocytes are all present before birth, arrested part way through meiosis, and released one per cycle. The ovarian cycle runs follicular, ovulation, luteal while the uterine cycle runs menstrual, proliferative, secretory, and the switch of oestrogen feedback from negative to positive produces the luteinising hormone surge that causes ovulation. The corpus luteum lasts 10 to 12 days unless human chorionic gonadotropin from an implanted blastocyst rescues it. Organogenesis occupies the first eight weeks and growth the remaining thirty, and labour is driven by the oxytocin positive feedback loop. Bottom line: endometriosis is normal tissue in an abnormal place responding normally to hormones, which is why the treatments that work are the ones that quieten the cycle.

Sources

1. OpenStax. (2022). 27.2 Anatomy and physiology of the ovarian reproductive system. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022). 27.1 Anatomy and physiology of the testicular reproductive system. In Anatomy and Physiology 2e. Rice University. openstax.org
3. OpenStax. (2022). 28.2 Embryonic development. In Anatomy and Physiology 2e. Rice University. openstax.org
4. Consoli, R. J., and Carlson, K. (2026). Endometriosis. In StatPearls. StatPearls Publishing. NCBI Bookshelf

12:45 on a Tuesday: 480 kilocalories and half a litre of air

A cheese and tomato sandwich on wholemeal bread, with a glass of milk. Read the labels and add them up: 60 grams of carbohydrate, 20 grams of protein, 18 grams of fat. Run lesson thirteen's arithmetic over that, 4 kilocalories per gram for carbohydrate and for protein and 9 for fat, and you get 240 plus 80 plus 162, so about 480 kilocalories. You swallow the first bite at 12:45. At 12:46 you take an ordinary breath, 500 millilitres of air, and do not notice it.

Those two events have to meet. The sandwich carries carbon and hydrogen locked into polymers. The breath carries oxygen. The only place in your body where the two are allowed to react is inside a mitochondrion a few micrometres long, on the far side of about eight organ systems, and the whole apparatus you have spent sixteen lessons on exists to get them into the same compartment and to carry the products away afterwards.

This lesson is that trace, timed. Each heading is a clock reading and a place. Nothing new is introduced except the metabolism at the centre; everything else is a number you already have, checked at the moment it does its work.

12:44, before anything is swallowed: the cephalic phase

The trace starts before the food does. Cephalic comes from the Greek kephale, head, and the cephalic phase is the part of digestion triggered by the sight, smell and taste of food rather than by food arriving anywhere. Signals from the eyes, nose and tongue reach the hypothalamus and the medulla, and the response travels out along the vagus nerve, the tenth cranial nerve and the main parasympathetic supply to the gut.

Saliva is already flowing. The stomach is already making acid. The pancreas has already begun a small release of enzyme. This is why the mouth waters at a smell, and it is a straightforward piece of feedforward control: a system that waits for a disturbance before acting is always behind, so the body uses the arrival of information to get ahead of the arrival of matter.

12:45:00 to 12:45:10, the mouth and the oesophagus: the last voluntary step

Teeth cut and grind, which changes nothing chemically and multiplies the surface area enormously. Salivary amylase starts on the starch, and the sixty-second experiment from lesson thirteen is the proof: chew plain bread long enough and it turns sweet, because the enzyme has cut the tasteless starch into sugars the tongue can detect.

The tongue shapes a bolus and pushes it back, and at that moment control passes out of your hands for good. The swallowing reflex, organised in the medulla, raises the soft palate to close the nasal cavity, folds the epiglottis over the larynx, and starts a peristaltic wave. Nothing you do consciously touches this meal again until it leaves. Worth pausing on: of the roughly sixteen hours the sandwich spends inside you, you control about two seconds of it.

12:46, one breath: 500 millilitres of which 350 count

Now the other half of the trace. Your diaphragm contracts, driven by the phrenic nerves that leave the spinal cord at levels C3, C4 and C5, and flattens downward. Thoracic volume rises, pressure inside the lungs drops below atmospheric, and 500 millilitres of air move in.

About 150 millilitres of that never reaches an exchange surface: it is the anatomical dead space of lesson twelve, sitting in nose, trachea and bronchi. So 350 millilitres of fresh air reaches the alveoli. Air is 20.9 percent oxygen by volume, so those 350 millilitres carry about 73 millilitres of oxygen.

How much of it do you keep? At rest an adult consumes about 250 millilitres of oxygen a minute and produces about 200 millilitres of carbon dioxide a minute (Patel and Bhardwaj, 2023). At 12 breaths a minute, that is about 21 millilitres of oxygen taken up per breath and about 17 millilitres of carbon dioxide added. Now do the exhaled gas. You breathed in 500 millilitres containing 104 millilitres of oxygen; you removed 21; so 83 millilitres of oxygen leave in 500 millilitres of exhaled air, which is 16.7 percent. That is why rescue breathing works: exhaled air is still four fifths as rich in oxygen as the air in the room. And the 17 millilitres of carbon dioxide in 500 makes exhaled air about 3.3 percent carbon dioxide, against 0.04 percent in the atmosphere, a rise of about eighty times.

Key idea: the ratio of those two numbers has a name. Carbon dioxide made divided by oxygen used, 200 over 250, is 0.8, the respiratory quotient. It is 1.0 for pure carbohydrate, 0.7 for pure fat and about 0.9 for protein, so 0.8 is the signature of a body burning a mixture. The gas leaving your mouth reports what you are burning.

12:45 to about 4:00, the stomach: a metering device with a brake

The bolus arrives in seconds and then stops moving for hours. Parietal cells hold the contents between pH 1.5 and 3.5, which unfolds the protein of the cheese and converts pepsinogen to pepsin. The stretch of the wall and the peptides appearing in it make G cells release gastrin, which drives yet more acid: a short positive loop that ends when the pH falls far enough to switch gastrin off.

What matters for the trace is the pyloric sphincter. Chyme is let through in small squirts, and the rate is not set by the stomach alone. The duodenum applies a brake, and the brake exists because the 18 grams of fat in this meal would overwhelm the digestive capacity downstream if they arrived at once. So the sandwich is metered out over the next few hours, which is also why a large meal keeps you feeling full long after you have stopped eating.

About 1:15, the duodenum: two hormones do the mixing

The name is arithmetic: duodenum from the Latin duodeni, twelve each, because it is about twelve fingerbreadths long. Into its first 25 centimetres come three streams, and two hormones released from its own wall call for them.

Secretin is released when acid arrives. It tells the pancreas to send bicarbonate, which lifts the pH from about 2 to about 7 or 8, and the reason is blunt: every pancreatic enzyme is denatured by stomach acid, so unless the acid is neutralised first, none of the rest of digestion happens at all. Cholecystokinin is released when fat and protein arrive. Its name is its job list, from Greek chole (bile), cyst (bladder) and kinin (to move): it squeezes the gallbladder, so bile salts arrive to emulsify the cheese fat, and it calls for the pancreatic enzymes. It also slows gastric emptying, which is the brake described above.

Coordinating all of this locally is the enteric nervous system, a few hundred million neurons embedded in the gut wall that can run peristalsis and secretion with the vagus cut. The brain modulates the gut; it does not operate it.

About 1:30, the brush border: the road forks

Maltase, sucrase, lactase and the peptidases make the final cut in the membrane of the absorbing cell itself, and the monomers cross at the point where they are made. Then the two nutrient streams separate, and the separation is one of the most consequential facts in the whole course.

Sugars and amino acids enter the capillary inside each villus, join the hepatic portal vein, and go to the liver before anywhere else. Fat does not. Reassembled inside the cell and packaged with protein, it is too bulky for a capillary, so it enters the lacteal, travels as lymph, and enters the bloodstream at the thoracic duct under the left collarbone, having bypassed the liver's inspection entirely.

Meanwhile something is happening that has nothing to do with absorption. Cells scattered along the intestinal wall detect the nutrients passing and release incretins, chiefly GIP and GLP-1. These reach the pancreatic beta cells through the blood and amplify insulin release. The result, the incretin effect, is that glucose swallowed produces considerably more insulin than the same quantity of glucose injected into a vein, because the gut has told the pancreas what is coming before the glucose itself arrives. Lesson nine mentioned drugs that mimic a gut hormone to amplify insulin release after eating. This is the hormone, and this is the moment.

About 1:40, the liver: the first pass, and the decision

Here is the problem the liver is solving. You carry roughly 5 litres of blood at a normal fasting glucose of about 90 milligrams per decilitre, which is 0.9 grams per litre, so the entire circulating glucose pool is about 4.5 grams. The sandwich supplies 60 grams of carbohydrate. That is more than thirteen times the whole pool, arriving over a couple of hours into a system that must not let the concentration run away.

Insulin, already primed by the incretins, does three things at once. In muscle and fat it moves GLUT4 transporter proteins from storage vesicles into the cell membrane, so cells that were nearly closed to glucose open. In the liver it drives glucose into glycogen. Everywhere it favours building over breaking down. Blood glucose peaks and comes back, and insulin release falls off: the negative feedback of lesson one, with a hormone as the effector.

The amino acids get a different treatment. What the body needs for protein synthesis is taken. There is no store for the surplus, so the liver strips the nitrogen off it by deamination, literally the removal of the amine group, and burns or converts the carbon skeleton left behind. Hold that thought: the nitrogen has to go somewhere, and where it goes is the second exit later in this lesson.

About 1:45, the delivery run: 4.9 litres a minute into an 8 micrometre tube

Glucose is now in the blood leaving the liver, and oxygen is in the blood leaving the lungs. Both take the same route. The left ventricle ejects a stroke volume of 70 to 80 millilitres per beat; at 70 beats a minute that is about 4.9 litres a minute, so your entire blood volume passes the heart roughly once a minute at rest. Pressure leaves the ventricle at about 120 millimetres of mercury and almost all of it is spent in the arterioles, which is exactly why arterioles are the control valves: dilate the ones feeding a working muscle and the same cardiac output is redirected there without the heart doing anything different.

At the capillary, the tug of war from lesson eleven. Hydrostatic pressure of about 35 millimetres of mercury at the arterial end, falling to about 18 at the venous end, against a colloid osmotic pull of about 25 that does not change. Fluid leaves at the start carrying dissolved glucose and amino acids, and is drawn back at the finish carrying waste. Oxygen and carbon dioxide are not part of that bargain at all: they cross by diffusion down their own partial pressure gradients, which is why they keep moving even where the pressures balance. The capillary is about 8 micrometres wide and a red cell is about 7.5, so every cell touches the wall as it squeezes past.

Inside the cell: where the sandwich and the breath finally meet

Cellular respiration happens in two compartments, and the split is worth holding because it explains why anything needs mitochondria at all.

In the cytoplasm: glycolysis. From Greek glykys (sweet) and lysis (loosening): the splitting of sugar. One six-carbon glucose is broken into two three-carbon pyruvate molecules. Four ATP are made and two are spent, a net gain of two, plus two molecules of the electron carrier NADH. No oxygen is required for any of it, which is why a sprinting muscle can run on glycolysis alone for a short time and pay for it in lactate afterwards.

In the mitochondrion. The name is Greek again, mitos (thread) and chondrion (grain), from what they look like under a microscope. Pyruvate crosses in and is converted to two-carbon acetyl coenzyme A, releasing one carbon as carbon dioxide immediately. Acetyl CoA enters the citric acid cycle, which strips the remaining carbons off as carbon dioxide and loads the electrons onto more NADH and FADH₂.

Then the payoff. Those carriers hand their electrons to the electron transport chain in the inner mitochondrial membrane. The energy released as electrons fall from complex to complex is used to pump hydrogen ions out of the matrix, building a gradient, and the ions flow back through an enzyme that uses their return to make ATP. At the end of the chain sits oxygen, and its entire job is to accept the spent electrons. Electrons, oxygen and hydrogen ions combine, and the product is water (Ahmad et al., 2023).

So what?: oxygen supplies no energy. It is the last seat in the queue, and without it the queue stops moving, the carriers stay loaded, and everything upstream halts within seconds. That is the whole reason you breathe.

How much ATP? Anatomy and Physiology 2e gives a net total of 36 ATP for each glucose fully oxidised (OpenStax, 2022a). Do the same accounting with measured per-carrier yields instead, 2.5 ATP per NADH and 1.5 per FADH₂ (Ahmad et al., 2023), and ten NADH plus two FADH₂ plus four made directly gives about 32, or 30 if the cytoplasmic NADH is ferried in by the cheaper of the two shuttles. So the honest answer is 30 to 36 depending on the accounting, and the reason it is a range rather than a constant is that some of the hydrogen ion gradient leaks back without making anything. Every textbook constant in this course has a measurement behind it.

The sandwich as stoichiometry

Now put numbers on one meal. The equation for the carbohydrate is one line: C₆H₁₂O₆ plus 6 O₂ gives 6 CO₂ plus 6 H₂O.

Glucose has a molar mass of 180, so 60 grams of carbohydrate is about one third of a mole. One third of a mole of glucose needs 2 moles of oxygen and produces 2 moles of carbon dioxide and 2 moles of water. Taking 22.4 litres as the volume of a mole of gas at standard temperature and pressure, that is 44.8 litres of oxygen consumed and 44.8 litres of carbon dioxide made, weighing 88 grams. The 2 moles of water weigh 36 grams, and that is metabolic water: water you did not drink, manufactured inside your mitochondria. And the ATP, at 36 per glucose, comes to about 12 moles. ATP has a molar mass of about 507, so those 12 moles weigh about 6 kilograms. You plainly do not contain 6 kilograms of ATP. Each molecule is spent and rebuilt many times a minute, which is what makes ATP a currency rather than a store.

The first exit: carbon, as a gas

Carbon dioxide made in the matrix diffuses out of the cell and into the blood, and lesson twelve already told you what happens to it. About 7 to 10 percent stays dissolved. About 20 percent binds to the protein part of haemoglobin. About 70 percent enters a red cell, where carbonic anhydrase combines it with water to make carbonic acid, which splits into a hydrogen ion and bicarbonate; the bicarbonate leaves for the plasma in exchange for chloride. At the alveolus the whole sequence runs backwards across a partial pressure gradient of only 5 millimetres of mercury, 45 in the blood against 40 in the alveolus, which works because carbon dioxide is about twenty times more soluble than oxygen in the fluid it must cross.

Here is the number that closes the loop. The carbohydrate of this one sandwich yields 44.8 litres of carbon dioxide. At a resting output of 200 millilitres a minute, breathing it all out takes 224 minutes: three and three quarter hours, almost exactly the time the meal takes to be absorbed and used. Some of the glucose is stored as glycogen first rather than burned at once, so the real figure is spread wider, but the order of magnitude is right, and it makes a point worth keeping. The mass you lose when you use up a store of carbohydrate or fat does not disappear and does not mostly leave in the toilet. It leaves through your mouth and nose, as gas, one breath at a time.

The same carbon dioxide also sets the pace of the breathing that removes it. Central chemoreceptors in the medulla read the pH of cerebrospinal fluid, which tracks blood carbon dioxide closely because the gas crosses the blood brain barrier freely. More carbon dioxide means lower pH there and harder breathing. No accounting is needed anywhere: the waste product is its own disposal signal.

The second exit: nitrogen, as urea

The 20 grams of protein in this meal contain nitrogen, and unlike carbon it cannot leave as a gas. Deamination in the liver releases ammonia, NH₃, which is toxic at concentrations far below those at which urea causes any trouble, and is particularly dangerous to the brain. So the liver converts it, in five enzymatic steps that begin in the mitochondria of the hepatocytes and finish in their cytoplasm: the urea cycle, which happens only in the liver (Barmore et al., 2023).

Count it. Protein is roughly 16 percent nitrogen by mass, which is where the factor of 6.25 used to estimate protein from nitrogen on food labels comes from. So 20 grams of protein carries about 3.2 grams of nitrogen. Urea is CO(NH₂)₂, molar mass 60, of which 28 is nitrogen. To carry 3.2 grams of nitrogen you need 3.2 times 60 divided by 28, which is about 6.9 grams of urea. Dissolve that in a day's 1.5 litres of urine and the concentration is around 4.6 grams per litre.

Urea leaves the hepatocyte into the blood and travels to the kidney, where lesson fourteen takes over. It is small, so it is filtered freely at the glomerulus along with everything else in the 180 litres a day. It is then partly reabsorbed, and the part that is reabsorbed is not wasted: urea contributes to the concentration of the medulla that the collecting duct draws water out against. Antidiuretic hormone decides how much water goes with it. The 180 litres becomes about 1.5, and the nitrogen from the cheese leaves the building.

The third exit: heat

The last exit is the one nobody lists as a waste product. About 60 percent of the energy released from nutrients appears as heat rather than as ATP (OpenStax, 2022b). Apply that to this meal: 480 kilocalories, of which about 288 becomes heat.

Is that a lot? Treat a 60 kilogram person as 60 kilograms of water, which needs 1 kilocalorie to warm each kilogram by 1 degree Celsius. Then 288 kilocalories would raise body temperature by about 4.8 degrees, and because tissue holds heat rather less well than water does, the true rise would be larger still. A body at 37.0 would reach 41.8, which is fatal. The heat has to leave as fast as it appears, and lesson one named the machinery that makes it leave: the hypothalamus comparing blood temperature with a set point, reducing sympathetic tone to skin arterioles so they dilate, and driving sweat glands when that is not enough. Anatomy and Physiology 2e gives the shares at rest as about 60 percent of heat lost by radiation, 20 by evaporation, 15 by convection and 3 by conduction (OpenStax, 2022b).

So the warmth you feel after a large meal is not the temperature of the food. It is the oxidation of the food, and a thermostat responding to it.

The ledger

Every row is a hand-off between two systems. The first column says who passes to whom, the second what actually crosses, the third a number from this course you can check, and the last what gives the order.

The whole trace on one page

Draw it, because the shape is the point. Take a landscape sheet. Along the bottom third, draw a long horizontal tube from left to right and label the stations in order: mouth, oesophagus, stomach, duodenum, small intestine, colon. Above the duodenum, draw two short arrows coming down into it, one labelled pancreas with bicarbonate and enzymes, one labelled gallbladder with bile salts.

From the small intestine, draw two arrows leaving upward, and make them different. Label the first hepatic portal vein, take it to a box labelled liver, and from the liver take a line to a circle in the middle of the page labelled heart. Label the second lacteal, take it up the left edge, past a node, and bring it into the heart line only at the very top, labelling that junction thoracic duct. That asymmetry is the fork in the road, and it should look wrong on the page, because it is.

Down the right-hand side, draw a vertical tube: nose, trachea, bronchi, one alveolus. Draw a short double-headed arrow between the alveolus and the heart circle and write 104 and 40 beside it, then 45 and 40 underneath for the return.

From the heart, draw one artery out to a small rectangle at the centre right and label it a cell. Inside it draw a bean shape and label it mitochondrion. Into that bean draw two arrows, one arriving from the gut line labelled glucose and one from the lung line labelled oxygen, and out of it three arrows: carbon dioxide going back to the alveolus, water staying put, and a wavy arrow labelled heat going off the edge of the page. Finally, draw one more arrow from the liver box down to a bean-shaped kidney at the bottom right, label it urea, and take a short line from the kidney off the page labelled urine.

Two things should now be visible that a list never shows. Everything converges on one bean, and everything leaves by three doors only: the lungs, the kidneys and the skin.

Where the chain gets cut

Here is the reason for building the trace. Each disorder in this course is not a separate topic. Each is a named cut at a named point in the chain you have just drawn, and the position of the cut predicts the symptoms and the treatment.

Read the last two rows together and the value of the trace becomes obvious. Kidney failure and liver failure both leave a patient full of nitrogen waste, and they look superficially similar. But one has failed at the disposal step and one at the conversion step, so in kidney failure urea is high and in liver failure urea is low while ammonia climbs. Same symptom, opposite numbers, because the cut is in a different place.

Common misconceptions

• Fat that you use up is burned away or passed out in the toilet. Follow the atoms. Carbon and hydrogen leave as carbon dioxide and water, and the great majority of the mass goes out through the lungs as gas. Weight loss is mostly exhaled.
• Oxygen gives you energy. Oxygen releases nothing. It sits at the end of the electron transport chain and accepts spent electrons, becoming water. Without it the chain backs up, which is why the shortage kills in minutes even though the fuel is still there.
• The body warms you by burning food the way a fire warms a room. The heat is real, but it is a by-product of chemistry that would happen anyway, and the body's problem is getting rid of it, not making it. One ordinary meal fully oxidised carries enough heat to raise core temperature past the point of survival if nothing removed it.
• Each organ system is a separate topic. No step in this lesson could be removed without stopping the next one. The systems are chapters in a textbook, not compartments in a body.

The takeaway

One meal of 480 kilocalories and one breath of 500 millilitres meet inside a mitochondrion, and everything between is transport and control. The cephalic phase starts secretion before food arrives, the swallow is the last voluntary act, gastrin drives acid while cholecystokinin applies the brake, and secretin neutralises the chyme so pancreatic enzymes can work at all. Brush border enzymes make the final cut, then sugars and amino acids take the portal vein to the liver while fat takes the lacteal and bypasses it, and incretins tell the pancreas what is coming before it arrives. Insulin moves GLUT4 into membranes and fills the liver with glycogen. A cardiac output of 4.9 litres a minute carries glucose and oxygen to a capillary 8 micrometres wide, where glycolysis in the cytoplasm and the citric acid cycle and electron transport chain in the mitochondrion release 30 to 36 ATP per glucose and hand the spent electrons to oxygen, which becomes water. One third of a mole of glucose yields 2 moles of carbon dioxide, 44.8 litres, about three and three quarter hours of resting breathing. The nitrogen of 20 grams of protein becomes about 6.9 grams of urea in the liver and leaves in 1.5 litres of urine. About 60 percent of the energy released becomes heat, which radiation, evaporation, convection and conduction must carry away as fast as it appears. The core of it: a meal and a breath enter, and carbon dioxide, urea, water and heat leave; every disorder in this course is a cut at one named link in that chain, and where the cut is tells you which number moves first.

Sources

1. OpenStax. (2022a). 24.2 Carbohydrate metabolism. In Anatomy and Physiology 2e. Rice University. openstax.org
2. OpenStax. (2022b). 24.6 Energy and heat balance. In Anatomy and Physiology 2e. Rice University. openstax.org
3. Patel, H., and Bhardwaj, A. (2023). Physiology, respiratory quotient. In StatPearls. StatPearls Publishing. NCBI Bookshelf
4. Ahmad, M., Wolberg, A., and Kahwaji, C. I. (2023). Biochemistry, electron transport chain. In StatPearls. StatPearls Publishing. NCBI Bookshelf
5. Barmore, W., Azad, F., and Stone, W. L. (2023). Physiology, urea cycle. In StatPearls. StatPearls Publishing. NCBI Bookshelf