Explain how animals maintain homeostasis using negative feedback, with reference to thermoregulation and osmoregulation

What this dot point is asking

SCSA wants you to apply the stimulus-receptor-control centre-effector-response model to real systems such as temperature, water balance and blood glucose. A strong answer names each component and explains how negative feedback reverses the original change.

Homeostasis and feedback

Homeostasis is the maintenance of a relatively constant internal environment despite changes outside the organism. Variables such as body temperature, blood glucose, water and ion concentration, and pH must stay within narrow ranges for enzymes and cells to function.

Most homeostatic control uses negative feedback: a change away from the set point triggers a response that reverses the change, returning the variable towards normal. The components are:

• Stimulus: a change in a variable.
• Receptor: detects the change.
• Control centre: processes information and coordinates the response (often the brain, especially the hypothalamus).
• Effector: a muscle or gland that carries out the response.
• Response: an action that reverses the original change.

Control systems: nervous and endocrine

Animals coordinate homeostasis with two systems. The nervous system uses fast, short-lived electrical signals along neurons for rapid responses. The endocrine system uses hormones (chemical messengers) released into the blood, producing slower but longer-lasting effects. The hypothalamus links the two, monitoring the internal environment and directing both nerve and hormone responses.

Thermoregulation

Mammals are endotherms that keep body temperature near a set point (about 37 degrees Celsius in humans). Thermoreceptors in the skin and hypothalamus detect temperature change, and the hypothalamus coordinates responses.

When body temperature rises:

• Blood vessels in the skin dilate (vasodilation), increasing blood flow to the surface so more heat is lost.
• Sweat glands secrete sweat; its evaporation removes heat.
• Behavioural responses such as seeking shade.

When body temperature falls:

• Blood vessels constrict (vasoconstriction), reducing heat loss at the surface.
• Skeletal muscles shiver, generating heat.
• Hairs are raised by erector muscles, trapping insulating air.
• The thyroid and adrenal glands can raise metabolic heat production.

Ectotherms (such as reptiles) rely mainly on behaviour, basking to warm and seeking shade to cool, because they cannot generate much internal heat.

Osmoregulation

Osmoregulation is the control of water and solute concentration in body fluids, managed by the kidneys and the hormone ADH (antidiuretic hormone).

When blood becomes too concentrated (for example after sweating or low water intake), osmoreceptors in the hypothalamus detect the rise in solute concentration. The pituitary gland releases more ADH, which makes the kidney tubules (collecting ducts) more permeable to water. More water is reabsorbed into the blood, producing a small volume of concentrated urine. Blood concentration falls back towards normal.

When blood is too dilute, less ADH is released, the tubules reabsorb less water, and a large volume of dilute urine is produced. Again the system corrects the deviation through negative feedback.

Blood glucose regulation

Blood glucose is controlled by the pancreas using two hormones in negative feedback.

• After a meal, blood glucose rises. The pancreas releases insulin, which makes cells take up glucose and the liver store it as glycogen. Blood glucose falls.
• During fasting, blood glucose falls. The pancreas releases glucagon, which makes the liver break down glycogen and release glucose. Blood glucose rises.

These two opposing hormones keep blood glucose within a narrow range.

Why narrow limits matter

Enzymes have optimum temperatures and pH; outside these the rate of reaction falls and proteins can denature. Cell volume depends on the water balance of body fluids, so large changes in solute concentration can cause cells to swell or shrink. Maintaining homeostasis therefore keeps cellular processes efficient and protects against tissue damage.