Control of body temperature (HT)
Humans are endotherms — they keep their core temperature near 37 °C regardless of the outside temperature. This optimum is crucial because enzymes denature above ~40 °C and slow down sharply below ~35 °C.
The thermoregulatory centre
The thermoregulatory centre in the hypothalamus monitors blood temperature. It also receives information from temperature receptors in the skin about the external environment. The hypothalamus then triggers responses through nerves and hormones.
This is a classic negative feedback loop: a rise in temperature triggers cooling responses; a fall triggers warming responses.
Cooling: when body temperature is too high
- Sweating. Sweat glands release water and salts onto the skin. As the water evaporates, it takes energy from the body — cooling it. This only works well when humidity is low.
- Vasodilation. The blood vessels supplying the skin capillaries (specifically the arterioles) widen. More blood flows close to the surface, transferring heat to the skin and then to the air by radiation. The skin looks flushed/red.
- Hairs lie flat. Erector pili muscles relax. Less air is trapped near the skin → less insulation → faster heat loss. (Less effective in humans than in furry mammals, but still in the spec.)
Warming: when body temperature is too low
- Shivering. Skeletal muscles rapidly contract and relax. Respiration in the muscles releases extra heat as a by-product. Some of this heat warms the blood.
- Vasoconstriction. Arterioles supplying the skin capillaries narrow. Less blood flows near the surface, so less heat is lost by radiation. Skin appears pale.
- Hairs stand on end (goosebumps). Erector pili muscles contract. Air is trapped between hairs → better insulation. Almost vestigial in humans.
- Sweating stops.
Important details (often examined)
- It is arterioles, not capillaries, that constrict and dilate. Capillaries cannot change diameter.
- Vasodilation does not mean blood vessels move to the surface — they're already there. Their diameter changes.
- Sweat itself does not cool you — its evaporation does. That's why a damp sweaty T-shirt feels colder when wind blows.
- All these mechanisms act together; you don't choose one over another.
Diagram you may have to draw
A simple negative-feedback diagram with:
- "Body temperature rises above 37 °C" → thermoregulatory centre detects → sweat glands + vasodilation + hairs flat → temperature falls → set point restored. Arrow returns to the start to indicate the loop.
⚠Common mistakes
- Saying capillaries dilate / constrict. Wrong vessel. Use arterioles supplying the skin capillaries.
- Saying sweat cools by being cold. Sweat is body temperature. The cooling effect is from energy transfer during evaporation.
- Mixing up vasodilation and vasoconstriction. Mnemonic: dilate = wide = warm escape; constrict = narrow = retain heat.
- Saying shivering "uses up energy", so we get cold. Wrong direction — shivering generates heat through respiration of glucose in muscles.
Links
Builds on B5.1 (homeostasis and negative feedback). Connects to B4.2 (respiration in muscles), B2.2 (the role of blood vessels) and B5.6 (hormonal coordination — adrenaline plays a small role too).
AI-generated · claude-opus-4-7 · v3-deep-biology