AQA GCSE Biology: Control of Body Temperature

This whole guide is Triple award only. The body keeps its core temperature steady so that its enzymes work at their best. This guide explains what monitors body temperature and the responses the body makes when it gets too hot or too cold. It is a clear example of homeostasis by negative feedback.

Monitoring temperature

Body temperature is monitored and controlled by the thermoregulatory centre in the brain.

• The thermoregulatory centre contains receptors sensitive to the temperature of the blood flowing through the brain.
• Temperature receptors in the skin also detect external temperature changes and send impulses to the thermoregulatory centre.

Two sets of receptors, measuring two different things. The ones in the brain read the core temperature, which is what actually has to be defended. The ones in the skin read the surroundings, which is an early warning: they let the body start responding to cold air before the core has had a chance to fall at all.

The target is around 37°C, and the reason is enzymes. Below it, reactions run too slowly; a few degrees above it, enzymes begin to denature and stop working altogether. Denaturing is permanent, which is why a high fever is dangerous in a way that being cold is not.

When the body is too hot

If the core temperature rises too high, the body loses heat.

• Vasodilation: the blood vessels supplying the skin capillaries widen, so more blood flows through them and more heat is transferred from the blood to the environment by radiation.
• Sweating: the sweat glands produce sweat, and as it evaporates from the skin it takes heat energy with it, cooling the body.

Two things are worth getting exactly right here, because both are commonly marked wrong.

The blood vessels do not move. They widen. Writing that capillaries "rise to the surface" or "move closer to the skin" is a standard way to lose the mark: nothing travels anywhere, the vessels supplying the skin simply dilate so more warm blood passes through them.

Sweat cools by evaporating, not by being wet. The mark is for the evaporation taking energy from the skin, so sweat that drips off has done nothing. That is also why a humid day feels worse: the air is already near saturation, sweat evaporates more slowly, and the cooling mechanism works less well even though you are sweating more.

When the body is too cold

If the core temperature falls too low, the body conserves and generates heat.

• Vasoconstriction: the blood vessels supplying the skin capillaries narrow, so less blood flows through them and less heat is lost.
• Shivering: the skeletal muscles contract and relax rapidly, which increases respiration in the muscle cells and releases more heat.
• Sweating stops, so no heat is lost by evaporation.

Shivering is the only one of the three that actually generates heat rather than saving it. The muscle movement itself is not the point: it is that contracting muscles must respire, and respiration is exothermic, so the extra heat is a by-product of the extra respiration.

Notice that the cold responses are the hot responses reversed. That is negative feedback doing its job in both directions, and both sets switch off once the core temperature is back to normal.

How this fits together

Temperature control is one of the body's nervous responses. The rest of Topic 5 focuses on the other coordination system, the hormones of the endocrine system, starting with the endocrine system guide.