Homeostasis in animals: WACE Year 12 Biology
“Explain how animals maintain homeostasis using negative feedback, with reference to thermoregulation and osmoregulation”
Homeostasis is the maintenance of a stable internal environment within narrow limits; animals achieve it through negative feedback systems in which a receptor detects change, a control centre coordinates a response, and an effector acts to return the variable towards its set point.
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.
Negative feedback counteracts a change and is the basis of homeostasis. Positive feedback amplifies a change and drives a process to completion (for example, oxytocin during childbirth). Most homeostatic systems are negative feedback.
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.
Trace the stimulus-response pathway
On a hot day a person's blood temperature rises. Thermoreceptors in the hypothalamus (receptor) detect this. The hypothalamus (control centre) sends signals to sweat glands and skin arterioles (effectors). Sweating and vasodilation (response) increase heat loss. Body temperature falls back towards the set point, and once normal, the receptors stop signalling. This switching off is the hallmark of negative feedback.
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.
Homeostasis: maintenance of a stable internal environment. Set point: the target value of a variable. Negative feedback: a response that reverses a change. Effector: a muscle or gland that produces the response.
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.
Saying the body "makes more heat to cool down." Heat-loss responses (vasodilation, sweating) reduce temperature; heat-gain responses (shivering, vasoconstriction) raise it. Match the response to the direction of change.
- Confusing insulin and glucagon
- Insulin lowers blood glucose; glucagon raises it.
- Forgetting the response switches off
- Negative feedback stops once the variable returns to the set point; a response that keeps running would be positive feedback.
- Writing that ADH increases urine output
- More ADH means more water reabsorbed and less, more concentrated urine.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2021-style6 marksUsing the stimulus-receptor-control centre-effector-response model, explain how negative feedback maintains homeostasis. Apply your answer to the control of body temperature when a person becomes too hot.Show worked answer →
A 6 mark answer needs the general model plus the applied thermoregulation example.
General model. A stimulus is a change in a variable away from its set point. A receptor detects the change and signals a control centre, which coordinates a response carried out by effectors. The response reverses the original change, returning the variable toward the set point; this opposing action is negative feedback.
Applied to overheating. Stimulus: body temperature rises above the set point. Receptor: thermoreceptors in the skin and hypothalamus detect the rise. Control centre: the hypothalamus. Effectors: skin arterioles and sweat glands. Response: vasodilation increases blood flow to the surface so more heat is lost, and sweating cools the body as sweat evaporates. Temperature falls back toward the set point, and the receptors stop signalling.
Markers reward each component of the model correctly named and matched to the heat-loss responses, plus the opposes-the-change definition.
WACE 2023-style5 marksExplain the difference between the nervous and endocrine systems in coordinating homeostasis, and explain why having both is an advantage to an animal.Show worked answer →
A 5 mark answer needs the contrast plus the benefit of having both.
- Nervous system
- Uses fast electrical impulses along neurons, giving rapid, short-lived, precisely targeted responses (e.g. withdrawing from heat).
- Endocrine system
- Uses hormones released into the blood, giving slower-onset but longer-lasting, more widespread responses (e.g. insulin acting on many tissues).
- Why both is an advantage
- Some changes need an immediate response (handled by the nervous system) while others need sustained, body-wide regulation (handled by hormones). Having both lets an animal respond quickly to sudden danger and also maintain steady longer-term control of variables such as blood glucose, covering a wider range of situations than either system alone.
Markers reward the speed and duration contrast and the point that the two systems suit different timescales of response.