Thermoregulation
This lesson covers how the human body maintains a constant core temperature of approximately 37°C — a process called thermoregulation. This is a key example of homeostasis and negative feedback for Edexcel GCSE Biology (1BI0) Topic 7.
Why Must Body Temperature Be Controlled?
The core body temperature must be maintained at approximately 37°C because this is the optimum temperature for the enzymes that control the body's chemical reactions.
| If Temperature Is Too High | If Temperature Is Too Low |
|---|
| Enzymes begin to denature — their active site changes shape permanently | Chemical reactions become too slow — metabolic rate decreases |
| Proteins unfold and lose their function | Cells cannot carry out vital reactions efficiently |
| Cell damage occurs | In extreme cases: hypothermia, organ failure |
| In extreme cases: heat stroke, organ failure, death | In extreme cases: death |
Exam Tip: The optimum temperature for enzymes is 37°C, NOT 37°C exactly. It is an approximation. Always say "approximately 37°C" or "about 37°C" in your answers.
The Thermoregulatory Centre
The thermoregulatory centre is located in the hypothalamus, a region at the base of the brain. It acts as the body's internal thermostat.
How the Thermoregulatory Centre Works
The thermoregulatory centre monitors body temperature using two types of input:
- Blood temperature — the hypothalamus has thermoreceptors that directly monitor the temperature of the blood flowing through it.
- Skin temperature receptors — receptors in the skin detect the external temperature and send nerve impulses to the hypothalamus.
Based on this information, the hypothalamus coordinates responses to either cool the body down or warm it up.
Response to the Body Being Too Hot
When the body temperature rises above the normal set point (~37°C), the following mechanisms are activated:
1. Vasodilation
Vasodilation is the widening (dilation) of blood vessels near the surface of the skin.
- The arterioles (small arteries) supplying blood to the skin surface widen (dilate).
- This allows more blood to flow through the capillaries near the skin surface.
- More heat is transferred from the blood to the environment by radiation.
- The skin appears flushed or red because more blood is visible near the surface.
IMPORTANT — Common Misconception: Blood vessels do NOT move closer to or further from the skin surface. The blood vessels stay in the same position. What changes is the amount of blood flowing through them. During vasodilation, more blood flows through the vessels near the surface; during vasoconstriction, less blood flows through them.
2. Sweating
- Sweat glands in the skin produce sweat, which is released onto the skin surface.
- Sweat is mostly water with some dissolved salts and urea.
- As the sweat evaporates, it absorbs heat energy from the skin, cooling the body down.
- This is why sweating is less effective in humid conditions — the air is already saturated with water vapour, so evaporation is slower.
3. Hairs Lie Flat
- Tiny erector muscles attached to hairs in the skin relax.
- The hairs lie flat against the skin surface.
- This reduces the insulating layer of trapped air next to the skin, allowing more heat to escape.
Response to the Body Being Too Cold
When the body temperature falls below the normal set point (~37°C), the following mechanisms are activated:
1. Vasoconstriction
Vasoconstriction is the narrowing (constriction) of blood vessels near the surface of the skin.
- The arterioles supplying blood to the skin surface narrow (constrict).
- Less blood flows through the capillaries near the skin surface.
- Less heat is lost to the environment by radiation.
- The skin may appear pale because less blood is visible near the surface.
- Blood is diverted to flow through deeper blood vessels, keeping the heat within the body's core.
2. Shivering
- Skeletal muscles contract and relax rapidly and involuntarily — this is shivering.
- These rapid contractions require energy from respiration, which releases heat as a by-product.
- This additional heat warms the blood and raises body temperature.
3. Hairs Stand Up (Goosebumps)
- The erector muscles attached to hairs in the skin contract.
- This pulls the hairs upright (causing "goosebumps").
- The upright hairs trap a thicker layer of insulating air next to the skin.
- This layer of air reduces heat loss from the skin surface.
- This mechanism is more effective in animals with thick fur than in humans, where body hair is relatively sparse.
4. Reduced Sweating
- When the body is cold, sweat production is reduced or stops.
- Less sweat on the skin means less evaporative cooling.
Summary of Thermoregulation Responses
| Response | Too Hot | Too Cold |
|---|
| Blood vessels (arterioles) | Vasodilation — widen to allow more blood near surface → more heat lost | Vasoconstriction — narrow to reduce blood near surface → less heat lost |
| Sweat glands | Produce more sweat → evaporation cools the skin | Produce less sweat → less evaporative cooling |
| Hairs | Lie flat → less insulating air trapped | Stand upright → more insulating air trapped |
| Muscles | No shivering | Shivering → rapid muscle contractions release heat |
| Skin appearance | Flushed / red | Pale |
Thermoregulation as Negative Feedback
Thermoregulation is a classic example of negative feedback:
When the body is too hot:
- Receptor: Thermoreceptors in the hypothalamus detect a rise in blood temperature.
- Coordination centre: The hypothalamus processes this information.
- Effectors: Sweat glands increase sweating; arterioles undergo vasodilation; erector muscles relax (hairs lie flat).
- Response: The body cools down, and temperature returns to ~37°C.
- Once normal temperature is restored, the cooling mechanisms are switched off.
When the body is too cold:
- Receptor: Thermoreceptors detect a fall in blood temperature.
- Coordination centre: The hypothalamus processes this information.
- Effectors: Skeletal muscles shiver; arterioles undergo vasoconstriction; erector muscles contract (hairs stand up); sweat production decreases.
- Response: The body warms up, and temperature returns to ~37°C.
- Once normal temperature is restored, the warming mechanisms are switched off.
Exam Tip: When answering thermoregulation questions, always identify the receptor (thermoreceptors in hypothalamus), coordination centre (hypothalamus), and effectors (sweat glands, blood vessels, muscles). Then describe the response and how it brings temperature back to normal.
When Thermoregulation Fails
Hypothermia
- Occurs when the core body temperature drops below 35°C.
- Symptoms: uncontrollable shivering, confusion, drowsiness, slow heart rate.
- Causes: prolonged exposure to cold, immersion in cold water, inadequate clothing.
- Treatment: gradual rewarming (warm blankets, warm drinks, warm environment).
Hyperthermia (Heat Stroke)
- Occurs when the core body temperature rises above 40°C.
- Symptoms: hot and dry skin (sweating may stop), confusion, rapid heart rate, unconsciousness.
- Causes: prolonged exposure to extreme heat, strenuous exercise in hot conditions, dehydration.
- Treatment: rapid cooling (cold water, ice packs, shade, fluids).
Summary