By the end of this lesson
- Explain sweating and evaporation.
- Explain vasodilation and vasoconstriction.
- Apply negative feedback to temperature regulation.
BIG QUESTION
How does skin help keep core body temperature close to its optimum?
Negative feedback alters evaporation and blood flow near the body surface.
Temperature is monitored and corrected
Thermoreceptors in skin and the hypothalamus provide information about temperature. The hypothalamus coordinates responses when core temperature differs from its set range.
The response opposes the change, so it is negative feedbackA control mechanism in which a change from the normal level triggers a response that reverses the change, returning the level to normal..
A hot body increases heat loss
Sweat glands secrete sweat. Its evaporation requires energy, cooling the skin and blood.
vasodilationWidening of the arterioles supplying the skin capillaries, so more blood flows near the skin surface and more heat is lost. widens skin arterioles, sending more blood through surface capillary networks so more heat transfers to the surroundings.
A cold body conserves heat
vasoconstrictionNarrowing of the arterioles supplying the skin capillaries, so less blood flows near the skin surface and less heat is lost. narrows skin arterioles, reducing blood flow near the surface and heat transfer. Sweat production falls.
Capillaries themselves do not perform the main constriction, and blood vessels do not physically move closer to the surface.
Skin temperature responses
Effectors alter evaporation and surface blood flow.
| Condition | Skin response | Effect |
|---|---|---|
| Too hot | More sweating. | Evaporation increases heat loss. |
| Too hot | Skin arterioles dilate. | More surface blood flow increases heat transfer. |
| Too cold | Sweating decreases. | Less evaporative heat loss. |
| Too cold | Skin arterioles constrict. | Less surface blood flow reduces heat transfer. |
Language that earns precision
- thermoregulation
- Maintenance of core temperature within a suitable range.
- vasodilation
- Widening of arterioles to increase downstream blood flow.
- vasoconstriction
- Narrowing of arterioles to reduce downstream blood flow.
- evaporation
- Liquid-to-gas change requiring energy.
Common misconceptions
Claim: Sweat cools simply because it is cold.
Correction: Evaporation removes thermal energy; unevaporated sweat cools less effectively.
Claim: Capillaries move nearer the skin.
Correction: Arteriolar diameter changes blood flow through existing capillary networks.
Claim: Shivering is a skin response.
Correction: It is rapid skeletal-muscle contraction that generates heat.
Match the answer to the command word
Name the response, then link it to the direction of heat transfer.
Write skin arterioles, not “veins move”.
Recall, apply, then justify
Explain how skin increases heat loss when the body is too hot.
Hint
Use sweating, evaporation, vasodilation and surface transfer.
Suggested marking points
- Sweat secretion increases.
- Evaporation removes thermal energy from skin.
- Skin arterioles dilate and surface blood flow increases.
- More heat transfers from blood to the surroundings.
Explain vasoconstriction during cold conditions.
Hint
Arteriole diameter → blood flow → heat loss.
Suggested marking points
- Skin arterioles narrow.
- Less blood flows through capillaries near the surface.
- Heat transfer to the surroundings decreases.
Explain why sweating is less effective in hot, humid air and identify a risk.
Planning hint
Consider the water-vapour gradient.
What a strong design includes
- Humid air already contains much water vapour.
- The gradient for evaporation from skin is reduced.
- Sweat evaporates more slowly.
- Less thermal energy is removed per unit time.
- Body temperature may continue rising.
- This increases risk of heat exhaustion or heat stroke.