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In the last lesson you met homeostasis and saw how blood glucose is held steady by negative feedback. This lesson applies exactly the same principle to two more vital conditions: body temperature and the water content of the body. First you will study thermoregulation — how the body keeps its core temperature near 37°C whether you are exercising in the heat or shivering in the cold — using the structures in the skin. Then you will see, at a whole-body level, how the amount of water in the blood is kept steady by the hormone ADH. Both are controlled by negative feedback, so the framework from the previous lesson carries straight over. This is part of Topic B3 of OCR Gateway Combined Science A.
By the end of this lesson you should be able to explain how the body responds to overheating and cooling, name the structures in the skin that control temperature, and describe how water balance is controlled by ADH through negative feedback.
This lesson mainly develops AO1 (recalling the skin structures and the role of ADH), with AO2 when you apply the negative-feedback model to work out the body's response to overheating, cooling or dehydration.
The human body works best at a core temperature of about 37°C — the temperature at which its enzymes are most active. If the body gets too hot, enzymes may begin to denature; if it gets too cold, reactions slow down too much. Keeping the temperature steady is therefore essential, and this is called thermoregulation.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain. This centre contains receptors that detect the temperature of the blood flowing through it. There are also temperature receptors in the skin that send impulses to the centre about the temperature of the surroundings. When the centre detects that the body is too hot or too cold, it triggers responses through effectors in the skin and muscles — a textbook example of negative feedback.
It is worth pausing on why a steady temperature matters so much, because this explains the whole point of thermoregulation. The reactions inside your cells are controlled by enzymes, and enzymes are sensitive to temperature. As the temperature rises towards about 37°C, reactions get faster; but if the temperature climbs much higher, the enzymes begin to denature — their shape changes so they can no longer work, and the reactions stop. If the body gets too cold, the enzyme-controlled reactions become too slow to keep the body working properly. So the body has a narrow band of temperature in which its chemistry runs well, and thermoregulation exists to keep it inside that band. The same logic ran through the blood-glucose lesson, and it is the reason homeostasis matters at all: it protects the delicate conditions that the body's enzymes — and therefore the body's cells — depend on.
The skin has three structures that matter for temperature control: sweat glands, blood vessels near the surface, and hairs controlled by tiny erector muscles.
The thermoregulatory centre detects that the blood is too warm and triggers responses to lose heat:
The thermoregulatory centre detects that the blood is too cool and triggers responses to conserve and generate heat:
| Condition | Blood vessels | Sweat | Hairs | Shivering |
|---|---|---|---|---|
| Too hot | Vasodilation (widen) | More sweat (evaporation cools) | Lie flat | No |
| Too cold | Vasoconstriction (narrow) | Less/no sweat | Stand up | Yes (releases heat) |
Exam Tip: A common misconception is that "vasodilation" means the blood vessels move closer to the surface — they cannot move. It means the vessels widen, so more warm blood flows through the capillaries already near the surface and more heat is lost. Use the words vasodilation (cooling) and vasoconstriction (warming) precisely.
Exam Tip: Be careful with sweating and shivering. Sweating cools you down (evaporation removes heat). Shivering warms you up (muscle contraction releases heat from respiration). Mixing these up loses easy marks.
Two of these mechanisms are worth understanding a little more deeply, because "explain" questions ask why they work, not just what happens.
Evaporation of sweat. Turning liquid sweat into water vapour needs energy. That energy is taken from the skin as heat, so the skin — and the blood flowing through it — is left cooler. This is why sweat only cools you effectively when it can evaporate: on a humid day, or under clothing that traps the moisture, sweat stays as a liquid, little heat is removed, and you feel hotter and stickier. It is the evaporation, not simply being wet, that does the cooling.
Changing the blood flow. Heat is carried around the body by the blood. When the skin's blood vessels dilate (widen), more warm blood flows through the capillaries close to the surface, so more heat passes from the blood to the surroundings and is radiated away. When the vessels constrict (narrow), less warm blood reaches the surface, so less heat is lost and the warmth is kept in the core, protecting the vital organs. Remember that the capillaries themselves do not move — it is the amount of blood flowing near the surface that changes.
During a hard run on a warm day, a student's core temperature starts to rise above normal. Describe and explain three ways the body responds to bring the temperature back down.
Step 1 — recall the "too hot" responses: vasodilation, more sweating, and hairs lying flat.
Step 2 — explain each in terms of losing heat.
Step 3 — link to homeostasis. These are triggered by the thermoregulatory centre as negative feedback; the responses counteract the rise and return the temperature towards normal.
Answer: the body responds by vasodilation (more heat radiated from the surface), increased sweating (evaporation removes heat) and flattening the hairs (less air trapped). Together, coordinated by the thermoregulatory centre, these lose heat and bring the core temperature back to normal.
Common error: writing only "the person sweats" without saying that it is the evaporation of the sweat that removes heat. No evaporation, no cooling.
The body also has to keep the amount of water in the blood steady. If the blood becomes too concentrated (too little water) or too dilute (too much water), water will move into or out of cells by osmosis, and the cells can be damaged. So the water content of the blood must be controlled — another example of homeostasis.
Water is gained by drinking and eating, and it is lost in several ways:
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