How Does Your Body Stay at 37 °C?

The cells of your body live in a liquid: the blood and the fluid around the cells. They need that liquid kept just right. Not too hot or too cold, with enough sugar and oxygen, not too much carbon dioxide, not too salty or too watery. Outside, the conditions change all the time; inside, they must not.

Keeping the conditions inside the body steady while the outside changes is called homeostasis (homeo = alike, stasis = standing still). You already know two examples:

• Mammals and birds keep their body temperature steady whatever the weather: they are warm-blooded (homeothermic), from Grade 10: Vertebrata.
• Insulin and glucagon keep your blood sugar steady, from Grade 9: Pancreas.

There are many more:

What is kept steady	Normal level (roughly)	What happens if it goes wrong
core body temperature	36.5–37.5 °C	below 35 °C: hypothermia; above 40 °C: heatstroke
blood sugar (glucose), fasting	70–100 mg/dL	too low: shaky, faint; too high for long: diabetes
water in the body	about 50–60% of an adult’s mass	dehydration; too much water dilutes the blood’s salts
carbon dioxide in the blood	kept within narrow limits	the blood turns too acid or too alkaline
blood pressure	about 120/80 mm Hg at rest	fainting, or high blood pressure

“Steady” doesn’t mean perfectly fixed. Your temperature is about half a degree lower early in the morning than in the late afternoon, and it rises a degree or more in hard exercise. Homeostasis keeps each condition within a range, and brings it back when it strays.

Many homes in Nepal have an electric water heater, a geyser. Inside it is a thermostat: a switch that measures the water’s temperature. When the water is colder than the temperature it is set to, the thermostat switches the heater on; when the water reaches that temperature, it switches the heater off. If someone runs off hot water and cold water flows in, the heater switches on again. Nobody has to watch it.

Your body controls its temperature, and almost everything else in the table, in the same way. Every such loop has the same parts:

1. A stimulus: a change away from the normal level (the body gets too warm).
2. A sensor (or receptor) that detects the change (nerve endings in the skin; nerve cells in the brain that sense the temperature of the blood).
3. A control center that compares what the sensor reports with the level the body is set to, the set point (for temperature, a part of the brain called the hypothalamus, set to about 37 °C).
4. Effectors that act (sweat glands, the blood vessels of the skin).
5. A response that cancels the change: the body cools back towards 37 °C.

As the response brings the temperature back, the sensor reports less and less of a change, and the effectors do less and less. The output of the loop has been fed back to its start, and it works against the change. That is why it is called negative feedback. Negative doesn’t mean bad: it means the loop pushes the opposite way to the change, whichever way that is.

You have met the first and last words in Grade 9, in the reflex arc: receptor → sensory nerve → spinal cord → motor nerve → effector Grade 9: Reflex action. A reflex is a one-off answer to a stimulus. A feedback loop goes round and round: the effector’s action changes the very thing the sensor measures.

Look at insulin and glucagon again with these words. After a meal the blood sugar rises (stimulus); cells in the pancreas sense it (sensor and control center in one); they send out insulin, which makes the liver and muscles take in sugar (effectors); the sugar falls back (response). When the sugar falls too low, a second loop sends out glucagon and pushes it up again. Many conditions are guarded by two loops that push in opposite directions, like a geyser that has both a heater and a cooler. That is why insulin and glucagon are called antagonistic hormones.

The control center for temperature is the hypothalamus, a part of the brain about the size of an almond, just above the pituitary gland Grade 9: Pituitary gland. It gets two kinds of report: from temperature sensors in the skin, which warn early that the outside is hot or cold, and from its own nerve cells, which feel the temperature of the blood flowing through it, the body’s core. It answers through nerves, mostly the autonomic nerves you met in Grade 9 Grade 9: Autonomic nervous system.

When you are too hot:

• Sweating. Sweat glands pour water onto the skin. As it evaporates it takes heat from the skin. This is the body’s most powerful way to lose heat, and in hot weather the only one: once the air is hotter than your skin, the air heats you instead of cooling you.
• Widening the blood vessels of the skin (vasodilation). More warm blood flows close to the surface, where it loses heat to the air. That is why a hot face turns red.
• Behavior. You look for shade, take off a layer, fan yourself, drink water, slow down.

When you are too cold:

• Narrowing the blood vessels of the skin (vasoconstriction). Less warm blood flows near the surface, so less heat escapes, and the skin of your fingers and toes turns pale and cold. The skin becomes a blanket wrapped round the warm core.
• Shivering. Muscles twitch rapidly, many times a second. They do no useful work, so the energy they use becomes heat. Hard shivering can make several times as much heat as your body makes at rest.
• Goosebumps. Tiny muscles pull the hairs upright. In a furry animal, like a yak or a cat, this traps a thicker layer of still air; in us, with so little hair, it does almost nothing. It is a leftover from our hairier ancestors.
• Behavior. You put on a jacket, curl up, sit by the fire, stamp your feet.

Behavior is part of homeostasis too, and for many animals it is the main part. A lizard has no sweat and no shivering: it keeps warm by lying in the sun in the morning and hiding in the shade at noon. A water buffalo has far fewer sweat glands than a cow, so on a hot day it lies in a pond or a mud wallow. A dog sweats only a little, through its paws, and cools itself mainly by panting: air moved fast over its wet tongue and mouth evaporates water there.

How much heat does sweat carry?

Your body is always making heat, because the chemistry in every cell gives out energy. Sitting still, an adult makes about 90 W of heat, a little less than an old 100 W light bulb. Walking, about 300 W; running hard, about 800 W. All of it has to leave the body, or you get hotter.

To evaporate 1 gram of sweat takes about 2,400 J of heat, taken from your skin. That is a lot: nearly six times what it takes to warm the same gram of water from 0 °C to 100 °C. You know from Grade 10 how to work out the heat needed to warm something, Q = mcΔθ Grade 10: Calculating heat. The human body is mostly water, and its specific heat capacity is about 3,500 J/(kg °C).

Now try being the hypothalamus’s patient. The model in the widget is simplified, but it follows the real physics: heat made by the body, heat gained or lost through the skin, and sweat that cools only as fast as the air lets it evaporate.

When you catch an infection, such as flu, typhoid or dengue, the body’s defenses send chemical messages to the hypothalamus that raise its set point, from 37 °C to, say, 39 °C. Nothing is wrong with the thermostat: it has been turned up on purpose, and many germs grow less well at the higher temperature.

Now think about what the loop does. The blood is still at 37 °C, but the set point is 39 °C, so the hypothalamus “thinks” the body is too cold. It narrows the skin’s blood vessels and makes you shiver, and you feel cold and pull the blanket up, even though your temperature is rising. This is the chill at the start of a fever.

When the infection is beaten, or you take a medicine for fever like paracetamol, the set point comes back down to 37 °C. Now a body at 39 °C is too hot for the thermostat: you sweat, your face flushes, you throw off the blanket. The fever has “broken”.

Negative feedback works only within limits. The sweat glands can make only so much sweat, and the muscles can shiver only so hard. Push the body past them and the loop loses control.

Hypothermia. On a mountain in wind and wet clothes, a body can lose heat faster than shivering can make it. Below 35 °C the person shivers hard, fumbles and stumbles; below about 32 °C shivering may stop, the person becomes confused and sleepy and may not realize they are in danger. Call for medical help, and meanwhile get the person out of the wind and wet, take off wet clothes, and wrap them in dry blankets or a sleeping bag with the head covered. Give a warm, sweet drink only if they are fully awake, never alcohol. Don’t rub their arms and legs or put them in a hot bath: warm them gently.

Heat exhaustion and heatstroke. In hard work in hot, humid weather, the body can make and gain heat faster than sweat can remove it, and it loses water and salt with every liter of sweat. Heat exhaustion comes first: heavy sweating, weakness, headache, dizziness, feeling sick. Rest in a cool, shaded place, loosened clothes, cool water sprayed or sponged on the skin with fanning, and plenty of water or Jeevan Jal usually bring a person back within half an hour; if not, treat it as heatstroke. If the core passes about 40 °C, the brain itself begins to fail: the person becomes confused, may stop sweating, may collapse. This is heatstroke, a medical emergency. Call for help and cool the person at once: wrap them in a cool, wet sheet or sponge them with cold water, and fan them. If they faint, lay them in the recovery position and check their breathing.

Heatstroke is dangerous because the loop flips. Normally, a hotter body sweats more and cools down. But when a hot brain stops controlling the sweat, a hotter body loses heat less well, so it gets hotter still. A change that makes itself bigger is the opposite of negative feedback.

There is a second kind of feedback loop. In positive feedback, the response makes the change bigger, not smaller, and the change grows faster and faster until something ends it. That is no use for keeping anything steady, but it is just right when the body needs something to happen completely and quickly:

• Childbirth. When a baby’s head presses on the opening of the womb, nerves signal the brain, and the pituitary releases a hormone, oxytocin, that makes the womb squeeze. The squeezing pushes the head harder, which releases more oxytocin, which makes the squeezing stronger. The loop stops when the baby is born and the pressing ends.
• Blood clotting. At a cut, the first platelets that stick to the wound release chemicals that make more platelets stick, and more clotting chemicals form. The clot builds up fast until the hole is closed.

Negative feedback	Positive feedback
The response works against the change.	The response makes the change bigger.
Brings a condition back to its set point.	Pushes it further and faster from where it started.
Keeps things steady: homeostasis.	Gets a job done completely and quickly.
Runs all the time.	Needs an end (the birth, the closed cut).
Body temperature, blood sugar, water, CO₂	Childbirth, blood clotting

The same idea runs other loops you can feel.

Water. When you lose water, by sweating or on a long walk without a drink, your blood becomes a little more concentrated. Sensors in the hypothalamus detect it. You feel thirsty (behavior), and the pituitary releases a hormone (ADH, the antidiuretic hormone) that makes the kidneys take water back from the urine they are making. Less water leaves, and your urine turns dark yellow. Drink a lot, and the opposite happens: plenty of pale urine. The color of your urine is a simple sign of this loop at work.

Breathing. Your muscles make carbon dioxide as they work. Sensors in the brainstem and in the big arteries of the neck detect when the CO₂ in the blood rises, and the brainstem makes you breathe faster and deeper, which removes it. Hold your breath, and after a while the urge to breathe becomes too strong to resist: it is driven mostly by the rising CO₂, not by the lack of oxygen.

Heart rate. You know from Grade 10 that your heart rate rises in exercise and falls back in rest Grade 10: Heartbeat, pulse rate or heart rate. Your heart rate isn’t kept steady for its own sake; the heart is an effector. What the body keeps steady is the oxygen and carbon dioxide in the blood and the blood pressure. When you start to run, your brain speeds up the heart through the sympathetic nerves at once, and sensors in the working muscles and blood vessels report how hard they are working, so the heart pumps faster and harder, bringing more oxygen and taking away more CO₂. When you stop, the parasympathetic nerves slow the heart, and the rate falls back to its resting level Grade 9: Nerves and hormones work together.

Heart rate is easy to measure, with nothing but your fingers and a clock. That makes it the best way to see homeostasis at work: a change (exercise), a response, and a return to rest when the change stops.

A good investigation starts with a question you can answer by measuring. For example: How does your heart rate change during and after exercise, and how long does it take to get back to its resting rate?

Then plan it before you start:

1. Make a prediction, with a reason. “My heart rate will rise during skipping, because the muscles need more oxygen, and fall back to its resting rate within a few minutes after I stop, because a feedback loop slows it when the need is gone.”
2. Decide on the variables. What you change on purpose is the independent variable: here, the time after exercise (or, in another investigation, how hard or how long you exercise). What you measure is the dependent variable: the heart rate. Everything else must stay the same, the controlled variables: the same exercise for the same time (say 1 minute of skipping), the same person, measured sitting down, the same way of counting, at the same time of day, not just after a meal or tea.
3. Decide how to measure, and how precisely. Counting the pulse for 15 seconds and multiplying by 4 is quick, which matters, because the rate starts falling as soon as you stop. But it gives only multiples of 4, so each reading may be up to about 4 beats a minute out. Counting for 30 seconds (× 2) is more precise but slower.
4. Repeat. Do the whole thing three times, resting fully between trials, and take the mean (average) at each time. One trial could be upset by a miscounted beat; three agreeing trials are evidence.
5. Keep it safe. Gentle exercise only, stop if you feel dizzy or unwell, and anyone with a heart or breathing problem should only do the counting.
6. Record it in a table, then plot it: time across, heart rate up.

A plant can’t look for shade or drink when it is thirsty, but it has feedback loops of its own. On the underside of most leaves are thousands of tiny pores, the stomata (one is a stoma, Greek for “mouth”). Through them the leaf takes in the carbon dioxide it needs for photosynthesis, and through them it loses water vapor, which pulls water up from the roots.

Each stoma lies between two curved guard cells. When the plant has plenty of water, the guard cells are swollen with it, and they bow apart and open the pore. When the plant is short of water, the guard cells go limp, and they close the pore. So when the soil dries or the air is very dry and hot, the stomata close and the leaf loses much less water: a response that works against the change, negative feedback again. (A plant hormone, abscisic acid, carries the message from dry roots to the guard cells; Grade 9 mentions it among the plant hormones.) The cost is that with its stomata closed, the plant takes in little carbon dioxide, so it grows little until the water comes back.

Plants respond to water in slower ways too. Roots grow more towards wet soil, and a plant grown in dry soil puts more of its growth into roots and less into leaves. These changes take days or weeks rather than minutes.

• How Does Food Become Energy, and Become You? Where the heat your body makes comes from: the chemistry that releases energy from food inside every cell.
• How Does a Gene Make a Protein? Insulin, ADH and oxytocin are proteins. How a cell reads its DNA to build them.
• How Does One Cell Become a Whole Person? How the cells of sweat glands, muscles and the brain all came from one, and became so different.