Thermoregulation: GCSE Biology

Why does the body need to regulate its temperature?

Body temperature is critical because all metabolic reactions are catalysed by enzymes, and enzymes have an optimum temperature — for human enzymes, approximately 37 °C. A rise above ~42 °C denatures enzymes (the active site changes shape, substrates no longer bind). A significant fall in core temperature slows enzyme-catalysed reactions, eventually stopping vital processes.

Human beings are endotherms (warm-blooded): we generate heat internally from cellular respiration and regulate body temperature using physiological mechanisms, rather than depending on external warmth.

What is the role of the hypothalamus?

The hypothalamus, a region of the brain, acts as the body's thermostat. It contains:

• Thermoreceptors that monitor the temperature of blood flowing through the brain (core temperature).
• A temperature set point of approximately 37 °C.
• Connections to the skin and muscles to trigger corrective responses.

The skin also contains thermoreceptors that detect surface temperature and send nerve impulses to the hypothalamus. When core temperature deviates from 37 °C, the hypothalamus sends signals via the nervous system and hormones to trigger corrective mechanisms.

What happens when the body is too hot?

When core temperature rises above 37 °C, the hypothalamus triggers responses that increase heat loss:

Sweating

Sweat glands in the skin produce sweat (mainly water). As sweat evaporates from the skin surface, it absorbs latent heat from the skin, cooling it. This is the most important mechanism for heat loss during exercise. Note: sweating only cools effectively if the atmosphere is not too humid — in high humidity, sweat cannot evaporate as readily.

Vasodilation

The arterioles (small blood vessels) supplying the skin widen (dilate). More blood flows close to the skin surface, increasing the temperature gradient between skin and environment and increasing heat loss by conduction, convection, and radiation. The skin appears flushed and pink.

Hairs lie flat

Erector pili muscles relax, so body hair lies flat against the skin. This reduces the insulating layer of air trapped among the hairs, allowing more heat to escape from the skin. (In humans with little body hair, this effect is minimal.)

What happens when the body is too cold?

When core temperature falls below 37 °C, the hypothalamus triggers responses that reduce heat loss and increase heat production:

Shivering

Skeletal muscles contract and relax rapidly and involuntarily. Muscle contractions release heat (from ATP hydrolysis in the muscle cells), raising core temperature. Shivering is metabolically expensive — it uses glucose rapidly.

Vasoconstriction

The arterioles supplying the skin narrow (constrict). Less blood flows close to the skin surface, reducing heat loss to the environment. The skin appears pale.

Hairs stand on end

Erector pili muscles contract, pulling hair follicles upright. This traps a thicker layer of air next to the skin, providing insulation. In other mammals with thicker fur, this is an effective insulator (piloerection). In humans, the result is "goose bumps" with little insulating benefit.

Increased metabolic rate

The hypothalamus also signals the adrenal glands and thyroid to release hormones (adrenaline, thyroxine) that increase the rate of cellular respiration in many body cells, generating more heat.

Summary table: responses to temperature changes

How is thermoregulation a negative feedback system?

Negative feedback is the mechanism by which a deviation from a set point triggers a corrective response that returns the system to the set point. In thermoregulation:

1. Stimulus: core temperature rises above 37 °C.
2. Receptor: thermoreceptors in hypothalamus detect the rise.
3. Effectors: sweat glands and arterioles activated.
4. Response: sweating and vasodilation reduce temperature.
5. Return to set point: when temperature reaches 37 °C, the responses switch off.

The same logic applies in reverse for a temperature drop. The word "negative" refers to the fact that the response opposes (negates) the original change.

Frequently asked questions

Why is 37 °C the optimum temperature for human enzymes?

Human enzymes have evolved over millions of years in an environment maintained at ~37 °C by internal metabolism. Natural selection has favoured individuals whose enzymes have active site shapes complementary to substrates at 37 °C, providing the highest reaction rates needed to sustain life processes. Many bacteria and archaea have enzymes with different optima (e.g. extremophiles in hot springs have enzymes optimised for 80–90 °C), but for humans and other mammals 37 °C represents the evolutionary equilibrium between warm enough for fast enzyme activity and cool enough to maintain a manageable energy cost of thermoregulation.

What is hypothermia and what physiological changes occur?

Hypothermia is defined as a core body temperature below 35 °C. As core temperature falls, all physiological and mental processes slow — coordination deteriorates, shivering intensifies and then eventually stops (the muscles run out of ATP), and cardiac arrhythmias become life-threatening below about 28 °C. Hypothermia is especially dangerous in the elderly because the efficiency of thermoregulation declines with age. Treatment involves slow external warming and, in severe cases, warm intravenous fluids.

How does exercise affect thermoregulation?

During vigorous exercise, muscles generate large amounts of heat from ATP hydrolysis — core temperature can rise rapidly. The hypothalamus detects this and triggers profuse sweating and marked vasodilation of skin arterioles. Blood is redistributed toward the skin for cooling. This is why athletes look flushed and sweat heavily during exercise. If exercise occurs in high heat and humidity, the cooling capacity of sweating is reduced, and core temperature may rise to dangerous levels (heat exhaustion → heat stroke above ~40 °C), which can be life-threatening.

What is the difference between ectotherms and endotherms in terms of thermoregulation?

Endotherms (birds and mammals) generate heat internally from metabolic reactions and use physiological mechanisms (sweating, shivering, vasodilation, vasoconstriction) to maintain a near-constant core temperature. Ectotherms (reptiles, fish, amphibians) rely on external heat sources and behavioural responses (basking in sun, moving into shade or water) to regulate body temperature. Endothermy requires much more food energy but allows activity in a wider range of environmental temperatures; ectothermy is more energy-efficient but restricts activity to warmer conditions.

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