Physiology: How Bodies Keep Themselves Alive

Run up a flight of stairs and four things happen without a decision. Your heart speeds up, you breathe harder, you start to feel warm, and soon you sweat. Walk back down, stand still, and all four drift back to where they began. Nothing is reading a dial; each is a loop that senses a number and pushes it back.

Physiology asks how a living animal does this, organ by organ. A physiologist looks at a body and sees a set of regulated quantities (blood pressure, blood oxygen, temperature, water, sugar) and the plumbing, muscles and chemical messengers that hold them in range. This piece takes the eight ideas that stay with people who have studied it. It describes how bodies work; it is not medical advice, and nothing here replaces a clinician who knows you.

Homeostasis: hold the number, accept the cost

In the nineteenth century Claude Bernard wrote that a complex animal lives in a stable "internal environment" even as the outside changes. Walter Cannon named the idea homeostasis in the 1920s and gave it a popular book, The Wisdom of the Body (1932). The mechanism is almost always negative feedback: a sensor measures the quantity, a control centre compares it with a set point, and an effector pushes it back toward the set point.

Two things follow, and they are the heart of control theory (see Control Systems, 34/20). A loop with finite gain never removes the error completely: a body in a cold room settles slightly below its set point. And a loop with a delay between sensing and acting overshoots; push the gain too high and it oscillates, like a shower where the hot tap answers ten seconds late.

With gain 4 and no delay the model settles near 33, short of 37, and it settles smoothly (the exact answer is (17 + 4 × 37) ÷ 5 = 33.0). Raise the gain to 15 and the target moves to (17 + 15 × 37) ÷ 16 = 35.75, a dip of only 1.25 degrees. Add a delay of just 2 steps and gain 15 runs away completely; even gain 4 keeps ringing with a delay of 4. Real regulation keeps gain high, stacks several loops and uses fast sensors, which is why healthy bodies are steady and why tired or sick ones can overshoot.

The heart and circulation: flow equals push over resistance

The heart is two pumps in series. The right sends blood through the lungs; the left sends it round the body. In each beat a ventricle ejects a stroke volume; times the number of beats a minute, that is the cardiac output. At rest a healthy adult is typically around 5 litres a minute, for example 70 beats of about 70 mL. Pushing blood round the body is like pushing water through pipes: the pressure you need is flow times resistance.

The fourth power is the striking part. Halve a tube's radius and resistance rises sixteen-fold. That is why the body steers blood by making small vessels a little narrower or wider (after a meal, more to the gut; in the cold, less to the skin), and why a small narrowing of an artery matters so much. Real vessels are elastic and blood is not a simple fluid, so Poiseuille's law is a guide, not a prediction.

At 70 beats × 70 mL the output is 4.9 L/min and pressure comes out near 88 mmHg. Narrow the small vessels by 10% and resistance rises by about 52% (1 ÷ 0.9⁴), pressure follows, unless the heart compensates. A fit person at rest has a bigger stroke volume and a slower rate for the same output.

Lungs and gas exchange: diffusion needs area and a gradient

Oxygen crosses from air to blood by diffusion alone. Fick's law says that flow of gas through a membrane is proportional to the area, the pressure difference of that gas, and inversely to thickness. So the lung is built as a huge surface a few cells thin: some 300 million tiny air sacs (alveoli) give a gas-exchange surface of tens of square metres (often quoted as about 70; estimates differ) wrapped in blood capillaries.

What drives the flow is the partial pressure of oxygen. Air is 20.95% oxygen at every altitude; what changes is the total pressure. Climb and the push to get oxygen into blood shrinks. The oxygen pressure in the lung's air sacs is lower than in the air you breathe, because you subtract carbon dioxide, which is why breathing harder helps:

At sea level the model gives an inspired oxygen pressure of about 19.9 kPa (149 mmHg) and an air-sac value near 100 mmHg. On a 4,000 m pass the inspired oxygen pressure is about 58% of sea level. At the height of Everest, standard-atmosphere pressure leaves an inspired value about 26% of sea level, and with normal breathing the air-sac value is below zero in this model: the only way to have any oxygen at all is to breathe much harder and blow off CO₂, which climbers do.

Kidneys: filter everything, take back almost all

The kidneys do not look for waste and pull it out. They do the opposite: about a million tiny filters in each kidney (nephrons) push a large share of blood plasma, water and small molecules, into a tube, and then the tube takes nearly all of it back. What is not taken back is urine. Normal filtration is around 125 mL a minute, about 180 litres a day, from a plasma volume of only about 3 litres: the blood is filtered some 60 times a day.

Because the filter is so large, a tiny change in the fraction taken back makes a large change in the output. A hormone from the pituitary, ADH (vasopressin), tells the collecting ducts how many water channels to open, and the nephron's loop, a hairpin that concentrates the fluid around it, sets how concentrated urine can get. Animals of dry habitats, such as desert rodents, have long loops and make far more concentrated urine than we can. When kidneys fail, dialysis imitates the filter; it is a hard job, which shows how much the kidney does.

At 125 mL/min and 99.2% reabsorbed, urine is 1.44 L a day. Move the fraction to 98.5% and it doubles to 2.7 L; to 99.6% and it halves to 0.72 L. A swing of half a percentage point in what is taken back is the whole difference between a thirsty day and a wet one.

Hormones: slow messages through the blood

Nerves are for speed, in milliseconds. Hormones are for broadcast and persistence: a gland releases a chemical into blood, which reaches every cell, and only cells with the right receptor respond. Ernest Starling coined the word in 1905; the hormone insulin was isolated in Toronto in 1921, and the 1923 Nobel Prize went to Banting and Macleod, who shared their money with Best and Collip.

The loops are the same negative feedback as before, just slower. The hypothalamus tells the pituitary, the pituitary tells the thyroid or adrenal gland, and the hormone released tells the hypothalamus to ease off. Blood sugar is a two-hormone loop: insulin from the pancreas helps cells take up glucose after a meal, and glucagon raises it between meals. A few loops run positive, so that one event feeds itself to a conclusion: the surge of luteinizing hormone that triggers ovulation, or oxytocin in labour.

How long a hormone acts is set by how fast it is cleared. Adrenaline is gone in minutes, which suits an emergency; thyroid hormone (thyroxine) has a half-life of about a week, so its level takes weeks to move and it is a slow tuner of metabolism. In general a level reaches 90% of a new steady value after about 3.3 half-lives, so a hormone with a long half-life cannot be quickly "reset" by anything. Popular "hormone balancing" claims in wellness ads usually ignore all of this: hormones are tied to feedback loops, so pushing one tends to be answered by the others.

Muscle: force and speed trade off

A muscle fibre is a stack of overlapping thick (myosin) and thin (actin) filaments. In 1954 two teams, Andrew Huxley with Rolf Niedergerke and Hugh Huxley with Jean Hanson, independently proposed the sliding filament model: the filaments keep their length, and myosin heads grab the actin and pull it past in repeated strokes, each powered by ATP and triggered by calcium. Muscle shortens because many tiny ratchets act in parallel and in series.

Because each stroke takes time, a fast shortening muscle has fewer heads attached at once: it can pull hard or fast, not both. A. V. Hill described the curve in 1938:

Power is force times speed, so it is zero at both ends (no speed when holding a maximal load, no force when moving freely) and peaks at about a third of the maximal force for typical constants. That is why a bicycle has gears: they let your legs work near the power peak.

With those constants, power peaks when the load is 31% of the maximum and the muscle is shortening at 31% of its fastest speed; the peak is about 9.5% of F₀ × v_max. Raise the load to 80% and speed falls to 5% of maximum.

Temperature control: a thermostat with a heater, a fan and a pipe

Mammals and birds hold the core near 37 °C (set points differ by species) by balancing heat made against heat lost. The hypothalamus compares the signals from the skin and core with a set point and drives the effectors: shivering and brown fat make heat; narrowing skin vessels keeps it in; widening them and sweating let it out. Sweat cools by evaporation, which needs the air to take up more water. Fever is not a failure of the thermostat but a raised set point: you feel cold until the body reaches it.

That cooling has a limit when the air is hot and wet. In 2010 Steven Sherwood and Matthew Huber argued that a sustained wet-bulb temperature of 35 °C is a theoretical limit for human survival. In 2022 laboratory work on young, healthy adults found people hitting their limit well below that, and the value depends on both temperature and humidity. The honest summary is: there is no single threshold, and older and ill people are at risk at lower values.

Scaling: Kleiber's law and the pace of small animals

If you double an animal's length, its mass goes up eight-fold, but the surface it loses heat through goes up only four-fold. Small animals lose heat faster for their weight, so they burn more per kilogram. In 1932 Max Kleiber compared animals from rats to cattle and found that basal metabolic rate rises not as the mass nor the surface (the 2/3 power) but about as the three-quarter power of mass.

A 70 kg person comes out near 82 W (about 1,700 kcal a day), a 25 g mouse about 0.2 W, an elephant of 4 tonnes about 1.7 kW. Per kilogram, the mouse burns around twenty times what an elephant does, and its heart races at several hundred beats a minute where the elephant's beats about thirty. West, Brown and Enquist proposed in 1997 that the exponent comes from the branching of the circulatory network; others (Kozłowski, Konarzewski) disagree, and data for small birds and insects often fit 2/3 better. Whether the exponent is exactly 3/4 is still argued over.

Slide to a 25 g mouse: about 0.2 W under the 3/4 law, against 0.4 W on the 2/3 line, and 8.6 W per kg against 1.2 for a person. Often-repeated claims that every mammal gets the same number of heartbeats in a lifetime are only roughly true; people, with a heart rate of about 70, clearly exceed it.

What a graduate just sees

• Everything regulated has a sensor, a set point and an effector; read any disease as a loop that has broken or been overloaded.
• Feedback with a delay and a lot of gain oscillates; the body needs fast sensors and several loops.
• Flow equals pressure over resistance, and resistance goes with radius to the fourth power, so a slightly narrower artery is a big deal.
• Oxygen delivery is diffusion down a pressure gradient across a very large, thin surface.
• The kidney filters everything and reabsorbs almost all, so tiny percentage changes make big volume changes.
• Hormones are slow because they are broadcast and cleared slowly; half-life sets the tempo.
• Muscle is a force–speed trade; maximum power sits at a modest load.
• Metabolic rate rises more slowly than mass, so small animals live fast.

Where it shows up next

• Biochemistry (35/43) and Cell Biology (35/44) sit below it: ATP, receptors and ion pumps are the parts of every loop.
• Biological Psychology (35/20) and Control Systems (34/20) use the same feedback logic for brains and machines.
• Microbiology (35/50): infection is a challenge physiology answers with fever, immunity and inflammation.
• In the world: sport and training, medicine and nursing, high-altitude expeditions, heat-wave planning, drug dosing, and wearables that track heart rate and sleep.

If you want to go deeper

• 📘 Textbook. Dee Unglaub Silverthorn, Human Physiology: An Integrated Approach (Pearson). A widely assigned textbook built around homeostasis and feedback.
• 📘 Textbook. Arthur Guyton and John Hall, Textbook of Medical Physiology (Elsevier). The classic medical-school reference, heavy on quantitative reasoning.
• 📘 Gentler. Bill Bryson, The Body: A Guide for Occupants (2019). A very readable tour with almost no maths.
• 🌐 Free course. OpenStax Anatomy and Physiology 2e, a free full textbook, and Khan Academy's Human anatomy and physiology videos.