You can walk out of a warm house into a freezing street and your core temperature will barely move. Your skin temperature drops a lot — that is why you feel cold — but the temperature deep inside you, where the enzymes are, stays close to 37 °C. This page is about how the skin, the muscles and one small part of the brain manage that.
📘 What you need to know
Thermoregulation is the control of internal body temperature, and it is a negative feedback system.
Temperature is detected by peripheral thermoreceptors in the skin and by receptors in the hypothalamus, which is the control centre.
When too hot: vasodilation, sweating, and hairs lie flat.
When too cold: vasoconstriction, shivering, hairs stand up, and more heat made by metabolism.
Uncoupled respiration in brown adipose tissue releases energy as heat instead of making ATP — non-shivering thermogenesis.
Thyroxin from the thyroid gland raises the basal metabolic rate, so more heat is produced.
Endotherms (mammals and birds) hold their body temperature steady using both physiological and behavioural mechanisms.
Why 37 °C and not 20 °C
Human enzymes have evolved to work best at about 37 °C, and moving away from that in either direction causes trouble — but for two completely different reasons.
Too cold: molecules have less kinetic energy, so they move more slowly. Collisions between enzyme and substrate become less frequent, fewer enzyme–substrate complexes form, and the rate of reaction drops. Nothing is destroyed; everything is just slow.
Too hot: the enzyme denatures. Vibration breaks the bonds holding the tertiary structure together, the active site changes shape, and the substrate no longer fits. This is permanent.
That asymmetry is why the graph below is not a symmetrical hill. On the left the line climbs steadily; on the right it falls off a cliff.
The steep right-hand side is the whole reason the body sweats so hard. Cooling can be undone later; denaturing cannot.
Detecting the change
The body checks temperature in two places at once, and this is worth getting right because questions often ask for both:
Peripheral thermoreceptors in the skin monitor the external temperature. There are separate receptors for heat and for cold. These give early warning — you start to react before your core has changed at all.
Receptors in the hypothalamus monitor the temperature of the blood, which reflects the core temperature.
Both send information to the hypothalamus, which acts as the control centre. It compares what it is receiving with the set point and sends instructions out to the effectors.
If a question asks “where is body temperature monitored?”, answering only “the skin” or only “the brain” usually costs a mark. Give both, and say which one measures external and which measures core.
What the skin actually does
Most of the response happens in the skin, because that is where heat is exchanged with the environment. Three things change in opposite directions depending on whether you are too hot or too cold.
The red band in each half is the same arteriole, drawn wide on the left and narrow on the right. Nothing has moved position; only the diameter has changed.
Responses to a rise in temperature
Vasodilation. The muscles in the walls of the arterioles supplying the skin relax, so the arterioles widen and more blood flows through the capillaries near the surface. More heat is then lost by radiation from the skin.
Sweating. Sweat glands secrete sweat onto the skin. As it evaporates it uses heat energy from the body to turn liquid water into vapour, which cools the skin.
Hairs lie flat. The hair erector muscles relax. Without an upright layer of hair, no insulating layer of air is trapped, and air can move freely over the skin carrying heat away.
Responses to a fall in temperature
Vasoconstriction. The muscles in the arteriole walls contract, narrowing the arterioles so less blood flows through the surface capillaries. Blood is diverted through deeper shunt vessels instead, so less heat is lost by radiation.
Shivering. Skeletal muscles contract and relax rapidly. The reactions powering this release heat, which warms the blood and raises core temperature.
Hairs stand up. The hair erector muscles contract, trapping a layer of air next to the skin. Air is a poor conductor, so this insulates. In humans it is nearly useless — we get goosebumps but not much insulation — because we have so little body hair compared with our ancestors.
A detail examiners love. Vasodilation and vasoconstriction happen in the arterioles, not the capillaries. Capillary walls are one cell thick and have no muscle, so they cannot change diameter at all.
Making more heat from the inside
Changing how much heat you lose only gets you so far. The body can also change how much heat it makes.
Uncoupled respiration in brown adipose tissue. Normally respiration is “coupled” to ATP production, so most of the energy released ends up in ATP. In brown adipose tissue this link is broken, so lipids are respired and all of the energy is released as heat instead of ATP. This is non-shivering thermogenesis, and it is especially important in newborn babies, who cannot shiver effectively.
Thyroxin and metabolic rate. The hypothalamus releases thyrotropin-releasing hormone, which makes the pituitary gland release thyroid-stimulating hormone, which makes the thyroid gland release thyroxin. Thyroxin raises the basal metabolic rate, and since most metabolic reactions release heat, more heat is produced. This is a slower, longer-term adjustment.
White adipose tissue does something different again — it sits under the skin and around the organs as a layer of stored lipid, working as insulation.
Notice the timescales. Vasoconstriction takes seconds, shivering takes seconds to minutes, thyroxin takes hours to days. If a question mentions someone moving to a cold country for a year, thyroxin is the answer it wants.
Putting it together as a feedback loop
Compare this with the blood glucose loop from the previous page. Same shape, different labels — that is the point of learning the pattern first.
Physiological and behavioural control
Endotherms are animals that keep a constant internal body temperature — mammals and birds. They do it in two ways, and the second one is easy to forget:
Behavioural mechanisms: moving into shade, going down a burrow, sunbathing, huddling, putting a jumper on.
Behaviour is cheap. Physiology costs energy. An animal will almost always try the behavioural option first, which is why a dog moves onto cool tiles before it starts panting hard.
Worked examples
WORKED EXAMPLE 1
Explain how the body returns core temperature to normal after it rises during exercise. [5]
Step 1: detection
Thermoreceptors in the skin and in the hypothalamus detect the rise in temperature.
Step 2: coordination
The hypothalamus acts as the control centre and sends impulses to the effectors.
Step 3: effector one — vasodilation
Muscles in the arteriole walls relax, so more blood flows near the skin surface and more heat is lost by radiation.
Step 4: effector two — sweating
Sweat glands secrete sweat; evaporation uses heat energy from the body, cooling the skin. Hair erector muscles also relax so hairs lie flat.
Step 5: close the loopCore temperature falls back towards 37 °Ca 5-mark answer needs detection, control centre, at least two effectors and the return to normal
WORKED EXAMPLE 2
Explain why newborn babies rely on brown adipose tissue to stay warm. [3]
Step 1: what they cannot do
Newborn babies cannot shiver effectively, so they cannot use muscle contraction to make heat.
Step 2: what brown adipose tissue does instead
In brown adipose tissue, respiration is uncoupled from ATP production, so lipids are metabolised and the energy is released as heat rather than being stored in ATP.
Step 3: name itNon-shivering thermogenesisthe word “uncoupled” is doing the real work here — use it
WORKED EXAMPLE 3
A student writes: “In the cold, the capillaries in the skin move deeper into the body so less heat is lost.” Identify two errors in this statement and correct them. [3]
Error 1: capillaries do not change diameter
Capillary walls are one cell thick with no muscle. It is the arterioles that constrict, because their walls contain muscle.
Error 2: nothing moves
Blood vessels do not change position. Blood is simply diverted through deeper shunt vessels while the surface arterioles are narrowed.
The corrected versionArterioles near the skin constrict, so less blood flows through surface capillaries and less heat is lost by radiationthis exact wording appears in mark schemes — it is worth memorising as a sentence
💡 Exam tip
Name the hypothalamus as the control centre every time. “The brain” is vaguer and sometimes not credited.
Give both sets of receptors: peripheral thermoreceptors in the skin (external) and receptors in the hypothalamus (core).
Use vasodilation and vasoconstriction by name, and say the arteriole muscles relax or contract.
For sweating, the mark is for evaporation using heat energy from the body, not just “sweat cools you down”.
Include a behavioural response if the question says “describe how a mammal regulates its temperature”.
Finish with the factor returning to normal — it is a negative feedback answer, so close the loop.
⚠ Common mix-up
Saying capillaries dilate or constrict. They have no muscle in their walls. Arterioles do the work.
Saying blood vessels “move up and down” in the skin. They stay put; only their diameter changes.
Thinking sweat cools you by being wet. The cooling comes from evaporation, which takes heat from the body. Sweat that drips off does nothing.
Muddling vasodilation with hair erection. Different structures: arteriole muscle versus hair erector muscle.
Treating too hot and too cold as equally bad. Overheating denatures enzymes permanently; cooling only slows reactions.
Forgetting thyroxin. Long-term cold adaptation is a metabolic-rate answer, not a shivering answer.
Writing that goosebumps keep humans warm. They are a leftover response — we have too little hair for it to help much.
That completes the homeostasis chain: the loop itself, then glucose, then temperature. If you can draw all three feedback diagrams from a blank page, you are ready for anything this sub-topic can throw at you.
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