Your core sits at about 37 °C whether you are in a heatwave or a snowstorm. That is not because your body is well insulated — it is because a set of effectors is constantly adjusting how much heat you make and how much you let go of.
📚 What you need to know
Thermoregulation is the control of internal body temperature, by negative feedback.
A stable core temperature keeps enzymes at their optimum — around 37 °C in humans.
Peripheral thermoreceptors in the skin monitor external temperature; receptors in the hypothalamus monitor internal temperature.
Endotherms (mammals and birds) maintain a constant internal temperature using physiological and behavioural mechanisms.
Responses to heat: vasodilation, sweating, hairs lie flat.
Responses to cold: vasoconstriction, shivering, hairs stand erect, uncoupled respiration in brown adipose tissue, raised metabolic rate via thyroxin.
Vasodilation and vasoconstriction happen in arterioles, never in capillaries.
Why 37 °C
Human enzymes have evolved to work fastest at around 37 °C, and both directions away from that are costly.
Too cold: molecules have less kinetic energy, so collisions between enzyme and substrate are less frequent and fewer enzyme-substrate complexes form. Reactions slow down or stop.
Too hot: enzymes denature. The tertiary structure is lost, the active site changes shape and enzyme-substrate complexes can no longer form at all.
The asymmetry is worth noticing. Cold slows you down and is usually reversible; heat destroys the enzyme and is not. That is why a fever of a few degrees is dangerous while a cold room is merely unpleasant.
Detecting the change
Peripheral thermoreceptors in the skin detect the temperature of the outside world. There are separate receptors for heat and for cold.
Receptors inside the hypothalamus detect the temperature of the blood, which is the core temperature.
Both send impulses to the hypothalamus, which coordinates the response.
Having both matters. Skin receptors give early warning — you start shivering when you step outside, before your core has actually cooled at all.
Responses to heat and cold
Humans have very little body hair, so the hair response does almost nothing for us. It is a leftover from ancestors with much thicker coats — which is why goosebumps feel useless. They are.
Responses to an increase in temperature
Vasodilation. Arterioles are the small vessels connecting arteries to the skin capillaries, and they have muscle in their walls. When those muscles relax, the arterioles widen and more blood flows through the skin capillaries. More blood near the surface means more heat is lost by radiation.
Sweating. Sweat glands secrete sweat onto the skin. It cools the skin by evaporation, which uses heat energy from the body to convert liquid water into water vapour.
Flattening of hairs. Hair erector muscles relax, so hairs lie flat. No insulating layer of air is trapped, and air can circulate over the skin and carry heat away.
Responses to a decrease in temperature
Vasoconstriction. The muscles in the arteriole walls contract, narrowing them so less blood flows through the surface capillaries. Blood is diverted through shunt vessels deeper in the skin, where it does not lose heat. Less heat is lost by radiation.
Erection of hairs. Hair erector muscles contract and hairs stand on end, trapping a layer of air that acts as an insulator.
Shivering. Muscles contract and relax rapidly. The metabolic reactions powering this release enough heat to warm the blood and raise core temperature.
Uncoupled respiration. In brown adipose tissue, respiration is uncoupled from ATP production, so all of the energy released is given off as heat rather than being used to make ATP. This is non-shivering thermogenesis, and it matters most in newborn babies, who cannot shiver.
Raised metabolic rate. Thyroxin from the thyroid gland increases the basal metabolic rate, and most metabolic reactions release heat.
You can answer almost any thermoregulation question by reading one half of this diagram out loud, swapping in the right effectors.
The hormonal route: thyroxin
The responses above are fast and nervous. There is also a slower hormonal route that changes how much heat you generate in the first place.
Three glands in a row, each releasing a hormone that acts on the next. Altering the thyroxin level alters how much heat your cells generate, which is a slower but longer-lasting adjustment than shivering.
Behaviour counts too
Endotherms do not rely on physiology alone. Behavioural mechanisms — moving into shade, sheltering in a burrow, sunbathing, curling up, putting a jumper on — change how much heat is gained or lost without the body spending any energy. They are often the first response, and the cheapest.
Where the heat actually goes. Heat always transfers from a hotter region to a cooler one. When your skin is warmer than the air, you lose heat to it. Vasodilation works by making your skin warmer, increasing the difference. Vasoconstriction works by making it cooler, reducing the difference.
Worked examples
WE 1
Explaining a response to cold
Explain how the body reduces heat loss when a person steps outside on a cold day. (4 marks)
Step 1: detection
The fall in temperature is detected by peripheral thermoreceptors in the skin, which send impulses to the hypothalamus.
Step 2: vasoconstriction
Muscles in the walls of the arterioles contract, so less blood flows through the surface capillaries and blood is diverted through shunt vessels. Less heat is lost by radiation.
Step 3: hairs
Hair erector muscles contract so hairs stand on end, trapping an insulating layer of air.
Step 4: state the outcome
Less heat is lost to the environment, so core temperature is maintained at about 37 °C.
Reduce loss first, then generate heat if that is not enoughthe question said reduce heat loss, so shivering is not the best answer here — shivering generates heat.
WE 2
Interpreting skin temperature data
A student’s skin surface temperature is measured before and after ten minutes in a warm room. It rises from 32.1 °C to 34.8 °C, while their core temperature stays at 37.0 °C. Explain these results. (3 marks)
Point 1: what caused the skin change
Vasodilation of the arterioles has increased blood flow through the skin capillaries, so warm blood from the core is brought closer to the surface, raising skin temperature by 2.7 °C.
Point 2: link to heat loss
A warmer skin surface increases the temperature difference with the surroundings, so more heat is lost by radiation.
Point 3: explain the constant core
Because this extra heat is lost, the core temperature is held at its set point, showing negative feedback is working.
Skin temperature is allowed to change so that core temperature does not have toa common error is saying the core temperature rose. Read the numbers — it did not.
WE 3
Brown adipose tissue in newborns
Newborn babies have a much higher proportion of brown adipose tissue than adults. Suggest why. (3 marks)
Point 1: what they cannot do
Newborns cannot shiver, so they cannot use muscle contraction to generate heat.
Point 2: what brown fat does instead
In brown adipose tissue, respiration is uncoupled from ATP production, so all of the energy released from metabolising lipids is given off as heat.
Point 3: why they need it more
Babies have a large surface area to volume ratio, so they lose heat quickly and need a way to generate it.
No shivering means non-shivering thermogenesis has to do the job“suggest” means you are not expected to have learned the answer — you are expected to apply what you do know.
💡 Exam tips
Say arterioles every single time you mention vasodilation or vasoconstriction.
For sweating, the mark is for evaporation using heat energy from the body, not just “sweat cools you”.
State whether the hair erector muscles contract or relax. Getting this backwards is a classic slip.
Distinguish reducing heat loss (vasoconstriction, hairs) from generating heat (shivering, brown fat, thyroxin). Questions often ask for one or the other.
Use thyroxin or thyroxine consistently — both spellings appear in IB materials and both are accepted.
⚠ Common mistakes
Saying capillaries dilate. They have no muscle in their walls. Only arterioles can change diameter.
Saying blood vessels move up and down in the skin. They do not migrate. The volume of blood flowing through them changes.
Saying sweat cools you as it is produced. It cools you as it evaporates. Sweat dripping off does nothing.
Confusing the receptor and the coordinator. Skin thermoreceptors detect; the hypothalamus coordinates.
Writing that shivering warms you because muscles are warm. The heat comes from the increased rate of metabolic reactions powering the contractions.
Up next: Osmoregulation & Excretion. We move to the kidney, which has two separate jobs: getting rid of toxic nitrogenous waste, and controlling how concentrated your blood is. Getting those two jobs straight in your head early makes the whole topic easier.
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