Homeostasis, feedback and thermoregulationEdexcel A-Level Biology A: Revision notes

Homeostasis is the maintenance of a stable internal environment, such as body temperature, within narrow limits. It is a dynamic equilibrium: values fluctuate slightly either side of a set point rather than staying perfectly fixed.

It matters because enzymes and cells only work well within narrow ranges. Too hot, and enzymes denature as hydrogen bonds break; too cold, and reactions slow because molecules have less kinetic energy.

Negative feedback reverses a change and returns the variable towards the set point, keeping it within narrow limits.

The loop is: stimulus (change) → receptor detects → coordinator (e.g. hypothalamus) → effector responds → change reversed.

Systems use antagonistic effectors (one raising, one lowering the value) so control works in both directions.

Positive feedback amplifies a change, driving the variable further from its starting value. It is not used to hold conditions steady, but is useful when a process needs to be completed quickly.

Examples: during childbirth, cervix stretch triggers oxytocin, which strengthens contractions and causes more stretch; the opening of sodium channels in an action potential; platelets releasing chemicals that attract more platelets in blood clotting.

Body temperature is held at about 37 °C. Thermoreceptors in the skin detect surface temperature, and thermoreceptors in the hypothalamus detect the temperature of the blood. The hypothalamus contains the thermoregulatory centre, which acts as the coordinator and sends impulses to effectors.

• Vasodilation: skin arterioles widen, more blood flows near the surface, so more heat is lost by radiation.
• Sweating: sweat glands secrete sweat; evaporation takes latent heat from the skin.
• Hairs lie flat: erector pili muscles relax, so less air is trapped.
• Behaviour: seeking shade, removing clothing.

During exercise, muscle respiration produces large amounts of heat, so these responses operate continuously. Heat lost balances heat produced and temperature stays steady (dynamic equilibrium).

• Vasoconstriction: skin arterioles narrow, blood flow to the surface falls, so less heat is lost.
• Shivering: rapid, involuntary contractions of skeletal muscle; extra respiration releases heat.
• Hairs raised: erector pili muscles contract, trapping an insulating layer of air.
• Sweating reduced: less evaporative heat loss.
• Behaviour: adding clothing, moving around.

Once the set point is reached the effectors switch off: negative feedback.