The big idea: Your core body temperature is held close to about 37 °C, even when the environment is hot or cold.
This is thermoregulation — a kind of homeostasis.
It is controlled by negative feedback: a change in temperature triggers a response that opposes the change and brings the temperature back to normal.
The control centre is the hypothalamus in the brain. The effectors are in the skin, the skeletal muscles and brown adipose tissue (brown fat).
Thermoregulation is one worked example of this loop: core temperature is the 'level', the hypothalamus is the control centre, and the skin/muscle/brown-fat effectors produce a response that opposes the change.
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- Thermoregulation
- Keeping the core body temperature close to a constant set point (about 37 °C) despite changes in the environment.
- Homeostasis
- Keeping the internal environment of the body steady, within narrow limits.
- Set point
- The 'target' value the body tries to hold — for human core temperature, about 37 °C.
- Negative feedback
- A control loop in which a change triggers a response that opposes (reverses) that change, returning the level to its set point.
- Hypothalamus
- The part of the brain that acts as the thermostat / control centre for body temperature.
- Effector
- A structure (e.g. skin arterioles, sweat glands, muscles, brown fat) that carries out the response to restore the set point.
Two branches — one loop: There is one loop but two branches:
Too hot → the body loses heat (vasodilation + sweating).
Too cold → the body conserves and generates heat (vasoconstriction + shivering + non-shivering thermogenesis).
The exam most often asks you to give the correct branch for a stated change — so always check first: is the body too hot or too cold?
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Read each branch as a chain of cause and effect: a change is detected, the hypothalamus signals the effectors, and the response reverses the change.
Match every effector to the heat it gains or loses — that is what scores the marks.
Too HOT → lose heat
- Receptors in the skin and hypothalamus detect the rise in temperature.
- Vasodilation: skin arterioles widen, so more warm blood flows near the surface → more heat is radiated/lost.
- Sweating: sweat glands release sweat; as it evaporates it takes heat from the skin → cooling.
- Core temperature falls back to the set point — the response opposed the rise.
Too hot: receptors detect the rise → the hypothalamus signals vasodilation and sweating → heat is LOST → temperature falls back to the set point. The response opposes the rise.
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Too COLD → conserve + generate heat
- Receptors detect the fall in temperature.
- Vasoconstriction: skin arterioles narrow, so less blood flows near the surface → less heat is lost (conserved).
- Shivering: skeletal muscles contract rapidly; the extra respiration in the muscles releases heat.
- Non-shivering thermogenesis: brown adipose tissue (brown fat) oxidises lipids to release heat directly, without movement.
- Core temperature rises back to the set point — the response opposed the fall.
Too cold: receptors detect the fall → the hypothalamus signals vasoconstriction, shivering and brown-fat thermogenesis → heat is CONSERVED and GENERATED → temperature rises back to the set point.
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Brown adipose tissue (a 1-mark favourite): Brown adipose tissue (brown fat) is specialised fat used to generate heat in the cold.
It carries out non-shivering thermogenesis: it oxidises (respires) lipids and releases the energy directly as heat, instead of using it to do work.
Because brown fat is so active when it warms the body, it shows up brightly on a PET/CT scan in cold conditions — the scan context the exam likes to use.
When too hot
- Vasodilation in the skin
- Sweating → evaporation cools
- Goal: lose heat
- Hairs lie flat (erector muscles relax)
When too cold
- Vasoconstriction in the skin
- Shivering generates heat
- Brown fat thermogenesis generates heat
- Goal: conserve + generate heat
| Feature | Vasodilation | Vasoconstriction |
|---|---|---|
| When it happens | When the body is TOO HOT | When the body is TOO COLD |
| What the arterioles do | Widen — more blood flows to the skin surface | Narrow — less blood flows to the skin surface |
| Effect on heat | More heat is radiated/lost from the skin | Less heat is lost (heat is conserved) |
| Restores the set point by | Cooling the body down | Keeping the body warm |
A memory hook: Dilate when you want to dump heat (both start with D — vasoDILATION when too hot).
Constrict to conserve heat (both start with C — vasoCONSTRICTION when too cold).
Shiver and brown fat = make heat; sweat and dilation = lose heat.
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How this is tested: On Paper 1A (multiple choice) you are very often given a stated change — temperature rises or falls — and asked to identify the body's response. Decide the branch first: hot → vasodilation + sweating; cold → vasoconstriction + shivering + brown-fat thermogenesis.
A 1-mark item may also ask the feedback type (always negative feedback) or how brown adipose tissue helps (non-shivering thermogenesis).
On Paper 2 a longer Explain can ask you to explain the mechanisms of thermoregulation — name the hypothalamus as control centre and pair each effector with the heat it gains or loses, all framed as negative feedback.
IB-style question — explain the response to overheating
After exercising in hot weather, a person's core body temperature rises above normal. Explain how the body returns its core temperature to the set point. [4]
How to score all four marks
- Detection / control centre. Temperature receptors (in the skin and hypothalamus) detect the rise; the hypothalamus acts as the control centre and signals the effectors. This is negative feedback.
- Vasodilation. Skin arterioles widen (vasodilation), so more blood flows near the skin surface and more heat is radiated/lost.
- Sweating. Sweat glands release sweat; as it evaporates it removes heat from the skin, cooling the body.
- Result. These responses oppose the rise, so core temperature falls back to the set point (~37 °C). (Award 1 mark per distinct point, up to 4.)
Final answer
Receptors detect the rise and the hypothalamus (control centre) signals effectors (negative feedback); vasodilation lets more heat be radiated from the skin; sweating cools by evaporation; together these lose heat so the temperature returns to ~37 °C.
Why this scores full marks: Each sentence is a separate, distinct point: detection by the hypothalamus, vasodilation, sweating, and the link back to the set point.
A 4-mark Explain needs four scoring ideas — and each effector must be paired with the heat it loses, not just named.
| Stimulus | Effectors | Response | Effect on heat |
|---|---|---|---|
| Too HOT (temp rises) | Skin arterioles | Vasodilation — more blood flows near the skin surface | MORE heat radiated/lost |
| Too HOT (temp rises) | Sweat glands | Sweating — sweat evaporates from the skin | Evaporation removes heat → cooling |
| Too COLD (temp falls) | Skin arterioles | Vasoconstriction — less blood near the surface | LESS heat lost (conserved) |
| Too COLD (temp falls) | Skeletal muscles | Shivering — rapid muscle contractions | Respiration in muscle generates heat |
| Too COLD (temp falls) | Brown adipose tissue | Non-shivering thermogenesis — brown fat oxidises lipids | Releases heat directly (no movement) |