Body temperature stays near its set point when heat gained and heat lost balance. Responses in the skin and muscles shift that balance one way or the other, and a strong answer explains both sides of it.
This skill applies the pattern from interpreting a negative-feedback loop to a real system. It leads to the glucose example in glucose regulation.
What are the competing effects?
Heat is gained from respiration in cells, especially muscles, and from a warm environment. Heat is lost from the skin surface by radiation and by the evaporation of sweat.
When the body is too hot, the responses increase heat loss: more sweat, and widening of skin arterioles. When the body is too cold, the responses reduce heat loss and raise heat production: narrowing of skin arterioles, less sweat, and shivering.
How do the skin responses compare?
| Response | When hot | When cold |
|---|---|---|
| Skin arterioles | widen (vasodilation) | narrow (vasoconstriction) |
| Blood flow near the surface | increases | decreases |
| Sweat | more produced | less produced |
| Muscles | no extra activity | shivering releases heat |
| Net effect | heat loss rises | heat loss falls, heat production rises |
Sweat cools through evaporation: water changing to vapour takes energy from the skin, which cools it.
Worked example
A student runs in warm weather, then rests in an air-conditioned room. Explain how her temperature is controlled at each stage. The temperatures are invented.
Stage 1, running. Muscle respiration releases extra heat, so core temperature rises from 37.0 °C to 37.5 °C. Receptors detect the rise. Sweat glands release more sweat and skin arterioles widen. Evaporation and radiation increase heat loss, which opposes the rise.
Stage 2, resting in a cold room. Her temperature is now 37.0 °C, but the cold air takes heat away. If it falls to 36.6 °C, receptors detect the fall. Skin arterioles narrow, sweating stops and muscles may shiver. Less heat is lost and more is produced.
Stage 3, return. Temperature rises towards 37.0 °C, the detector signal weakens and the responses ease.
Notice what changed. The same body, with the same set point, used opposite responses because the change was in opposite directions. That is the competing-effects idea: heat loss and heat gain are adjusted in turn.
The mistake to watch for
The common error is to say that vasodilation moves the blood vessels closer to the skin, or that it is the capillaries that widen.
Mistaken answer: “When hot, the capillaries move closer to the skin surface so heat escapes.”
Vessels cannot move through tissue. The widening happens in the arterioles that supply the skin capillaries.
The correction is to write: “arterioles supplying the skin widen, so more blood flows through the skin capillaries and more heat is lost by radiation.”
Check yourself
Try these, then open each answer.
1. Name the response in the skin when the body is too cold that reduces blood flow near the surface.
Show answer
Vasoconstriction: the skin arterioles narrow, so less blood flows near the surface and less heat is lost.
2. Explain how sweating cools the body.
Show answer
Sweat on the skin evaporates. Changing from liquid to vapour takes energy from the skin, so the skin and the blood near it lose heat.
3. Why does shivering raise body temperature?
Show answer
Muscle contraction needs energy from respiration, and some of the energy released is heat, which warms the blood and body.
Where this leads next
Next, apply the same loop logic to blood glucose in reading a glucose-regulation diagram. The mechanism and feedback builder can help you practise ordering the skin responses. If balancing two opposing effects in writing is slow for you, see how online one-to-one Biology tuition handles it.