Edexcel IGCSE Biology · Spec 2.93
Temperature Control
How the body keeps a constant core temperature — cooling down when too hot and warming up when too cold.
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Temperature Control
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Explained
Holding the body at 37 degrees
Human core temperature is kept at about 37 degrees Celsius, because that is the optimum for the enzymes controlling every reaction in the body. Too cold and reactions run too slowly; too hot and the enzymes denature, and that damage cannot be undone.
The brain monitors the temperature of the blood flowing through it, and receptors in the skin detect the temperature outside. When either moves away from normal, the brain triggers responses that push it back. That correction is negative feedback.
Getting rid of heat
Sweating. Sweat glands release sweat onto the skin, and it evaporates. Evaporation takes energy from the skin, so the body cools. Note that it is the evaporating that cools you, not the sweating, which is why a humid day feels so much worse.
Vasodilation. The blood vessels supplying the skin capillaries widen, so more blood flows near the surface and more energy is transferred to the surroundings by radiation. The skin looks flushed.
Hairs lie flat, so less air is trapped and insulation is reduced.
Conserving heat
Vasoconstriction. Those same vessels narrow, so less blood flows near the surface and less energy is lost. The skin looks pale.
Hairs stand up, trapping a layer of air next to the skin. Air is a poor conductor, so this insulates. In humans it produces goose bumps and achieves very little, but in a furry animal it works well.
Shivering. Muscles contract rapidly and repeatedly, which requires respiration, and respiration releases energy as heat. This is the only response that generates warmth rather than merely conserving it.
What vasodilation does not do
The blood vessels move neither closer to nor further from the skin surface. Capillaries cannot move. What changes is the diameter of the small arteries feeding them, which changes how much blood reaches the surface.
Writing that the capillaries move towards the skin is a common and specific error, and it is worth unlearning now.
What the examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a question asking what homeostasis means. It records an answer that gained no marks because the candidate referred specifically to thermoregulation rather than to homeostasis in general, and another that gained the mark for stating that homeostasis is the control of an internal environment.
Temperature control is one example of homeostasis, alongside blood glucose and water balance. Defining the general term by giving one instance of it does not answer the question.
A mark scheme on the same series makes the same point from the other direction, in a multiple choice question where maintaining body temperature is the correct example of homeostasis. Its explanations for the wrong options each say why that action is not restoring internal balance: lifting a weight is not, releasing saliva is not, withdrawing your hand is not.
The test to apply is whether the response returns some internal condition to its normal level. Pulling your hand from a hot object is a reflex, not homeostasis, even though both involve temperature.
Structuring the answer
Say what was detected, what response followed, and how that response changes the temperature.
The temperature of the blood rises, this is detected by the brain, sweat glands release more sweat, the sweat evaporates, and evaporation transfers energy away from the skin, so the temperature falls.
Five clauses, and each is normally a mark. Answers that name sweating and vasodilation without saying how either one removes energy are describing rather than explaining.
Spec 2.93
What you need to know
- Explain why we control body temperature
- Describe how the body cools down
- Describe how the body warms up
Active recall
Quick check
Answer each question before opening the answer.
How does the body respond when it is too hot?
Vasodilation (more blood to skin surface), sweating, and hairs lie flat — all increase heat loss.
How does the body respond when it is too cold?
Vasoconstriction, shivering, and hairs stand up to trap air — all reduce heat loss or generate heat.
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