50 Fun Facts About Physiology

Which gas building up in the blood, rather than a lack of oxygen, triggers the human urge to breathe?

Carbon dioxide is what the brainstem's sensors track, so free divers who over-breathe before a dive can black out underwater without ever feeling a need for air.

Which Russian physiologist won a 1904 Nobel Prize for work on digestion, not for his famous salivating dogs?

Ivan Pavlov was the first Russian Nobel laureate. He was measuring the digestive juices of dogs when he noticed them drooling before any food arrived, which led him to the conditioned reflex.

Which sea animal has three hearts and blue blood, coloured by a copper-rich protein instead of an iron-rich one?

Octopus blood is pumped by one main heart for the body and two smaller ones that push it through the gills. The main heart goes quiet whenever the animal swims.

Which gas from ordinary compressed air dissolves in a diver's tissues and forms the bubbles that cause 'the bends'?

Nitrogen makes up most of the air in a scuba tank and is not used up by the body. If a diver surfaces too fast, it comes out of solution like the fizz in a freshly opened bottle of soda.

Which lubricant inside a human joint holds the gas bubbles behind the pop of a cracked knuckle?

Synovial fluid takes about 20 minutes to reabsorb the gas, which is why the same knuckle will not crack twice in a row. Studies have found no link between the habit and arthritis.

Which nightly brain state brings the most vivid dreams while leaving nearly all of the body's muscles paralysed?

REM sleep shuts down the motor neurons so that dreamers do not act out their dreams. People whose paralysis fails can punch, kick or leap out of bed while still fast asleep.

Which human organ can regrow to its full size after surgeons remove more than half of it?

The liver is the only internal organ that can regenerate this way. In Greek myth the punished Prometheus has his eaten by a bird each day, only for it to grow back overnight.

Which hormone, released by glands sitting on top of the kidneys, drives the body's fight-or-flight response?

Adrenaline widens the pupils, speeds the heart and sends extra blood to the muscles within seconds. In the United States it is usually called epinephrine, from the Greek for 'upon the kidney'.

Which enzyme do most mammals stop making after weaning, though many adult humans keep it and can digest milk?

Lactase stays switched on for life in people who carry a mutation that spread once humans began dairy farming. Without it, milk sugar reaches the gut bacteria undigested.

Which African mammal needs about double a human's blood pressure, plus tight leg skin to stop blood pooling?

Giraffe leg skin holds blood in place so well that scientists have studied it as a model for astronaut and fighter-pilot suits. The high pressure is needed to reach a brain far above the heart.

Which word did physiologist Walter Cannon coin in 1926 for the body keeping its inner conditions steady?

Homeostasis joins the Greek words for 'similar' and 'standing still'. Cannon also gave English the phrase 'fight or flight', which he used for an animal's response to a threat.

Which almond-sized region of the human brain acts as the body's thermostat, triggering shivering and sweating?

The hypothalamus turns its own setting up during an infection, and that raised setting is what a fever is. It also controls hunger, thirst and the daily body clock.

Which iron-containing protein in red blood cells carries oxygen from the lungs and gives blood its colour?

Hemoglobin lets blood carry about 70 times more oxygen than could simply dissolve in its watery plasma. Each molecule can hold up to four oxygen molecules at once.

Which hormone from the pancreas lowers blood sugar by telling cells to absorb glucose after a meal?

Insulin was first injected into a patient in 1922, a 14-year-old boy who lay dying in a Toronto hospital. Until then there was no effective treatment for type 1 diabetes.

Which dome-shaped sheet of muscle beneath the lungs contracts and flattens each time a person breathes in?

The diaphragm also walls off the chest from the abdomen. A sudden involuntary spasm of it, followed by the vocal cords snapping shut, is what makes the sound of a hiccup.

Which English physician showed in 1628 that the heart pumps blood around the body in a continuous loop?

William Harvey worked out that the heart moves far more blood in half an hour than the body could possibly make, so the same blood had to be going round again. He was physician to two kings.

Which hormone does the pineal gland release in darkness to help set the body's sleep-wake cycle?

Melatonin production is held back by light, and blue light most of all, which is why a bright screen at bedtime can delay sleep. Travellers take it as a remedy for jet lag.

In which part of the human digestive tract, lined with finger-like villi, are most nutrients absorbed?

The small intestine is small only in width. Its folds and villi give it an absorbing surface of about 30 square metres, roughly the floor of a studio flat.

Which fifth basic taste did Japanese chemist Kikunae Ikeda identify in 1908 while studying seaweed broth?

Umami is the savoury taste of glutamate, strong in ripe tomatoes, mushrooms and aged cheese. The year after Ikeda identified it, it went on sale as the seasoning MSG under the brand name Ajinomoto.

Which nutrient does human skin make when sunlight hits it, and which children need to avoid rickets?

Vitamin D is the one vitamin that sunshine can supply: for most people the skin makes more of it than food provides. It lets the gut absorb the calcium that growing bones need.

Which gland hanging beneath the brain is nicknamed the 'master gland' because it directs so many other hormone glands?

The pituitary is only about the size of a kidney bean, yet the growth hormone it releases decides how tall a child will become.

Which butterfly-shaped gland in the neck needs iodine to make hormones that set how fast the body burns energy?

The thyroid can swell into a lump at the front of the neck, called a goitre, when a diet is short of iodine. That is one reason iodine is added to table salt.

Which 'love hormone' triggers contractions in childbirth and the let-down of milk during breastfeeding?

Oxytocin runs on a rare positive feedback loop: each contraction releases more of it, which makes the next contraction stronger, and so on until the baby is born.

Which tiny cell fragments in human blood clump together at a wound to begin forming a clot?

Platelets have no nucleus: they are chips that break off giant cells in the bone marrow. Each one circulates for only eight or nine days before it is replaced.

Which snail-shaped chamber of the human inner ear turns sound vibrations into nerve signals?

The cochlea sorts sound by pitch along its spiral. High notes are picked up near the entrance, where the membrane is stiff, and low notes travel on towards the far tip.

Which fatty sheath around nerve fibres speeds up their signals and is damaged in multiple sclerosis?

Myelin makes a nerve impulse hop from gap to gap along the fibre instead of travelling as one slow continuous wave. Being rich in fat, it is also what makes the brain's white matter white.

Which natural painkillers made in the brain bind the same receptors as morphine and are released during hard exercise?

Endorphins take their name from 'endogenous morphine', morphine made within. They were long credited with the runner's high, though blocking them in experiments does not stop it.

Which sea animal's giant nerve fibre let Alan Hodgkin and Andrew Huxley work out how nerve impulses travel?

Squid have a nerve fibre up to a millimetre and a half thick, wide enough for the pair to slide an electrode inside it. The work earned them the 1963 Nobel Prize.

Which word did Charles Sherrington introduce in 1897 for the junction where one nerve cell signals another?

Synapse comes from Greek words meaning 'to fasten together', and it was a classical scholar who suggested it. Most are in fact tiny gaps that a signal crosses as a burst of chemicals.

Which Italian scientist found in 1780 that a dead frog's leg muscles twitch when struck by an electrical spark?

Luigi Galvani called the effect animal electricity, and his name survives in the verb 'galvanize'. Mary Shelley knew of his experiments before she wrote Frankenstein.

Which brain chemical runs short in Parkinson's disease as nerve cells in the substantia nigra die off?

Dopamine cannot cross from the bloodstream into the brain, so it is no use as a pill. Patients are given L-DOPA instead, a precursor that does get across and is converted once inside.

Which chemical released by mast cells causes the itching, swelling and runny nose of an allergic reaction?

Histamine makes the smallest blood vessels leaky, which is what produces the swelling. The brain also uses it to stay awake, so older allergy pills that reach the brain cause drowsiness.

Which enzyme in human saliva starts breaking down starch while food is still being chewed?

Amylase is the reason plain rice or potato starts to taste sweet when it is chewed for a long time: some of the starch is being cut into sugar right there in the mouth.

Which strong chemical do cells in the human stomach lining secrete to kill microbes and help digest protein?

Hydrochloric acid would attack the stomach itself, so the same lining also makes a thick barrier of mucus. The acid's other job is to switch on the protein-cutting enzyme pepsin.

Which wave of muscle contraction and relaxation propels swallowed food along the human digestive tract?

Peristalsis squeezes food along in one direction, a ring of muscle tightening behind each mouthful. Earthworms use a similar travelling wave of muscle to crawl.

Which process draws water across a cell membrane toward the side holding more dissolved salt or sugar?

Osmosis is why a cell dropped into salt water loses water and shrinks. The same pull draws the moisture out of potato slices left in a strong salt solution.

Which heart test, with waves labelled P, QRS and T, won Dutch doctor Willem Einthoven a 1924 Nobel Prize?

The electrocardiogram machine Einthoven built weighed some 600 pounds and took five people to run. Patients sat with both arms and one leg in buckets of salt water, which served as the electrodes.

Which microscopic filtering unit, about a million of which sit in each human kidney, turns blood into urine?

A nephron filters the blood and then takes back almost everything useful. Together they produce about 180 litres of filtrate a day and reabsorb roughly 99% of the water in it.

Which motor protein forms the thick filaments that haul on actin to make a muscle fibre shorten?

Myosin heads grab the thin filament, pull and let go, and letting go costs energy in the form of ATP. After death the ATP runs out, the heads stay locked on, and the body stiffens into rigor mortis.

Which 'hunger hormone', released mainly by an empty stomach, rises before meals and falls after eating?

Ghrelin was discovered only in 1999. It does more than drive appetite: it also plays a part in the sleep-wake cycle, the sense of taste and the way the body handles glucose.

Which cluster of cells in the wall of the right atrium acts as the human heart's natural pacemaker?

The sinoatrial node sets the heartbeat by firing on its own, usually 60 to 100 times a minute at rest, with no order needed from the brain.

Which soapy substance coating the lungs' air sacs stops them collapsing and is often lacking in premature babies?

Surfactant lowers the surface tension of the wet lining so the sacs can inflate easily with every breath. Babies born too early have not yet made enough, and can be given a dose as treatment.

Which chemical did Otto Loewi find in 1921 slowing a frog's heart, the first proof that nerves signal with chemicals?

Acetylcholine came to Loewi in a dream: he woke on Easter Saturday night with the frog-heart experiment in his head. It is also the chemical that nerves use to make skeletal muscles contract.

Which French physiologist coined 'milieu intérieur' for the stable inner environment the body's cells live in?

Claude Bernard wrote a vaudeville comedy and a five-act drama before he turned to medicine. He went on to show that the body can make and store its own sugar.

Which gut messenger did Bayliss and Starling discover in 1902, prompting Starling to coin the word 'hormone'?

Secretin showed that organs can signal one another through the blood rather than through nerves. It tells the pancreas to release bicarbonate, which neutralises the acid arriving from the stomach.

Which stretch sensors in the carotid arteries and aortic arch keep the brain informed of blood pressure?

Baroreceptors can trigger a correction in less than a heartbeat. They are what stops blood pressure from collapsing when a person stands up and gravity pulls blood toward the legs.

Which term names the half litre or so of air an adult moves in and out with each quiet breath?

Tidal volume is only a small part of what the lungs can hold: an adult man's total lung capacity is about six litres. It rises by 30 to 40 percent during pregnancy.

Which chemical messenger makes the kidneys hold on to water, and is released less after drinking alcohol?

Antidiuretic hormone, also called vasopressin, is held back when alcohol is in the blood. The kidneys then let more water go, which is part of why drinking means so many trips to the bathroom.

Which kidney hormone tells bone marrow to make more red blood cells and became notorious as a cycling doping drug?

Erythropoietin output climbs when the body is short of oxygen. That is the idea behind altitude training: thin mountain air prompts an athlete's body to build extra red cells of its own.

Which tiny muscles at the base of each body hair contract in the cold to raise goosebumps?

Arrector pili muscles pull each hair upright. On a furry animal the raised coat traps a layer of insulating air, but on thinly haired humans the same reflex leaves only bumps.