💦 Sweating
The sweat glands release sweat onto the surface of the skin. The sweat is mostly water. As the water evaporates, it takes heat energy from the skin. This transfers heat away from the body and cools you down.
Co-ordination and Response » Skin and Temperature Regulation
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 2.93
- How the skin helps to keep your body temperature steady
- What happens in the skin when you are too hot: sweating and vasodilation
- What happens in the skin when you are too cold: vasoconstriction, shivering and hairs
- Why this is a good example of homeostasis
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Unlock This CourseKeeping your temperature steady
Your core body temperature stays close to 37°C, whether you are in a hot classroom or standing at a cold bus stop. You met this idea in the lesson Stimulus, Response and Homeostasis: homeostasis is keeping the internal conditions of the body steady. Temperature control is one of the best examples, and the skin plays a big part in it.
It works like every other response you have studied. A change is detected, a message is sent, and something acts to put things right.
👁️ Receptors
Temperature receptors in the skin detect changes in the temperature around you. Others in the brain monitor the temperature of the blood.
🧠 Co-ordinator
The brain receives the information and decides what to do. It sends nerve impulses to the effectors.
💪 Effectors
Sweat glands, the blood vessels in the skin and the muscles carry out the response that brings the temperature back to normal.
Key terms:
- Vasodilation: the widening of the blood vessels that supply the skin, so more blood flows through it.
- Vasoconstriction: the narrowing of the blood vessels that supply the skin, so less blood flows through it.
Why the skin matters
The skin is the largest organ on the outside of the body, so it is where most heat is lost to the air. It contains sweat glands, a network of blood vessels and temperature receptors. Heat is carried around the body in the blood. If more blood flows close to the surface, more heat can escape from it. If less blood flows near the surface, the heat stays deeper inside the body. The skin can therefore speed up or slow down heat loss, depending on what the body needs.
It is important to keep temperature steady because enzymes work best at an optimum temperature, as you saw in the lesson Temperature Effects on Enzymes. If the body gets much too hot or too cold, the reactions in cells cannot work properly.
When you are too hot
On a hot day, or after running, your blood gets warmer. The brain detects this and the skin goes into cooling mode. Two things happen.
🌡️ Vasodilation
The small blood vessels (arterioles) that carry blood to the skin surface widen. More blood flows close to the surface, so more heat is lost to the surroundings by radiation. This is why your face looks red and feels hot after exercise.
Both of these responses increase heat loss. As the body cools, the receptors detect that the temperature is back to normal, and the responses ease off. This is called negative feedback: the response reverses the change that started it.
When you are too cold
On a cold day the skin does the opposite. The aim now is to lose as little heat as possible, and to make a bit more.
❄️ Vasoconstriction
The blood vessels that supply the skin surface narrow. Less blood flows near the surface, so less heat is lost to the surroundings. This is why your skin looks pale and feels cold in the winter.
🚫 Sweating stops
The sweat glands release little or no sweat. This means there is no evaporation cooling the skin.
In one line each: the muscles may contract and relax very quickly, which is shivering, and this makes extra heat. The hairs on your skin can be pulled upright by tiny muscles, giving goose bumps. In animals with thick fur, this traps a layer of air which helps to keep them warm.
Compare: hot and cold
| Too hot | Too cold | |
|---|---|---|
| Sweating | Lots of sweat, which evaporates | Little or none |
| Blood vessels to the skin | Vasodilation (widen) | Vasoconstriction (narrow) |
| Blood flow near the surface | More | Less |
| Heat loss | Increases | Decreases |
| Other | Skin looks red | Shivering, skin looks pale |
Learn this
Too hot: sweat evaporates, vasodilation, more heat lost.
Too cold: vasoconstriction, no sweat, less heat lost.
Receptors in the skin and brain, the brain as co-ordinator, and the sweat glands and blood vessels as effectors.
Worked example
A student sits in a warm room and feels the temperature rise. Which words describe what her skin does? Her sweat glands release sweat, and the water in it evaporates and takes heat from her skin. The blood vessels to her skin widen (vasodilation), so more blood flows near the surface and more heat is lost. Both changes bring her temperature back down.
Worked example 2
A hiker waits at the top of a snowy hill. Explain what happens in her skin. The receptors detect the cold and the brain sends impulses to the effectors. The blood vessels to her skin narrow (vasoconstriction), so less blood flows near the surface and less heat is lost. Her sweat glands release almost no sweat, so there is no cooling by evaporation. Her muscles may also shiver to make extra heat. Together these keep her core temperature close to 37°C.
Common mistakes
- Saying that the blood vessels "move up towards the skin" when you are hot. They do not move. They widen or narrow.
- Writing that sweat cools you because it is cold. It cools you because the water evaporates and takes heat energy away.
- Mixing up the two words. Vasodilation means widening, so more heat is lost. Vasoconstriction means narrowing, so less heat is lost.
- Forgetting to say that the effect is to bring the temperature back to normal.
Exam-style question
(a) Name the process in which the blood vessels supplying the skin become narrower. [1 mark]
(b) Explain how sweating helps to reduce body temperature. [2 marks]
(c) Describe the changes in the skin that help a person to cool down on a hot day, and explain why they work. [3 marks]
Model answer
(a) Vasoconstriction (1)
(b) Sweat is released onto the skin and the water evaporates (1), which takes heat energy from the skin and so cools the body (1)
(c) Sweat glands release more sweat which evaporates (1), the blood vessels to the skin widen (vasodilation) (1), so more blood flows near the surface and more heat is lost to the surroundings (1)
Exam tip
Always say what the blood vessels do (widen or narrow), what that does to the blood flow, and what that does to heat loss. Naming the process alone is not enough.