Explain the concept of homeostasis and the role of negative feedback in maintaining a stable internal environment, including stimulus, receptor, control centre, effector and response

What this dot point is asking

QCAA expects you to define homeostasis around a set point and explain the negative feedback loop with all five components (stimulus, receptor, control centre, effector, response). This dot point underpins every other Unit 2 Topic 1 dot point.

The answer

Homeostasis is the maintenance of a relatively constant internal environment despite changes in the external environment. It is the foundational concept of animal physiology.

What "constant" actually means

Internal conditions are not held perfectly constant. Each regulated variable (body temperature, blood pH, blood glucose, blood osmotic pressure, blood oxygen) oscillates within a narrow range around a set point. Homeostasis is a dynamic steady state, not a fixed value.

Examples of human set points:

• Core body temperature: around 37 degrees Celsius.
• Blood pH: 7.35 to 7.45.
• Blood glucose: 4 to 6 mmol per L between meals.
• Blood osmotic pressure: around 300 mOsm per kg.

If a variable drifts too far from its set point, enzymes denature, membrane transport fails and cells die. The body therefore needs control systems that detect departures and trigger corrective responses.

The five components of a feedback loop

Every homeostatic control loop contains the same five components in the same order.

1. Stimulus. A change in the variable away from the set point (a rise or fall).
2. Receptor (sensor). A specialised cell or structure that detects the change. Examples: thermoreceptors in skin and hypothalamus, osmoreceptors in hypothalamus, beta cells detecting blood glucose, chemoreceptors detecting blood pH and CO2.
3. Control centre. Integrates input from the receptor and decides on a response. Often the hypothalamus, brainstem or an endocrine cell.
4. Effector. Carries out the response. Usually a muscle or a gland.
5. Response. The action that returns the variable toward the set point.

The response then feeds back to the receptor, which compares the new value to the set point and adjusts again.

Negative feedback

In a negative feedback loop, the response opposes the change that triggered it.

• A rise in a variable triggers a response that lowers it.
• A fall triggers a response that raises it.
• The variable oscillates around the set point.

Negative feedback is the dominant control mechanism in physiology. It is responsible for:

• Thermoregulation (sweating, shivering, vasomotor changes).
• Blood glucose control (insulin and glucagon).
• Osmoregulation (ADH and the kidney).
• Blood pressure (baroreceptor reflex).
• Hormone levels (the hypothalamic-pituitary-target gland axes, where rising target-organ hormone inhibits releasing hormone from the hypothalamus).

Positive feedback

In positive feedback, the response amplifies the change. The variable accelerates away from the starting value until an external event terminates the loop. It is rare in physiology because it is unstable, and is reserved for processes that need to go to completion.

Examples:

• Childbirth. Cervical stretch triggers oxytocin release, oxytocin causes contractions, contractions stretch the cervix further. Ends with delivery.
• Blood clotting. A small clot recruits more platelets and clotting factors. Ends when the wound is sealed.
• Action potential. Sodium influx depolarises the membrane, opening more sodium channels. Ends when channels inactivate.

Worked example: thermoregulation in cold

When core temperature climbs above the set point, the heat-loss centre activates the opposite effectors (sweat glands, vasodilation), restoring the set point.

Open and closed control

Homeostatic mechanisms are sometimes split into:

• Behavioural. Voluntary actions like seeking shade or putting on a coat.
• Physiological. Involuntary internal adjustments (sweating, hormone release).

Both contribute. A lizard regulates temperature mainly behaviourally (basking, sheltering); a mammal regulates mainly physiologically.

Common traps

Confusing the set point with the actual value. The variable oscillates around the set point, not at it.

Forgetting the control centre. QCAA mark schemes require all five components named in sequence.

Calling positive feedback a problem. It is unstable in general, but the body uses it deliberately for processes that need to amplify and terminate, like clotting and childbirth.

Saying homeostasis means "no change". Homeostasis is a dynamic steady state. Levels change constantly; they are corrected toward the set point.

Cross-link to Year 12 assessment

This dot point underlies all of Unit 2 (thermoregulation, osmoregulation, blood glucose, immunity) and reappears as a foundational concept in Unit 3 ecosystem dynamics (negative feedback also stabilises populations and ecosystems). EA Paper 1 short-response questions on Unit 2 routinely demand the five-component loop applied to an unseen scenario.

In one sentence

Homeostasis is the maintenance of internal conditions in a stable steady state around a set point, achieved by negative feedback loops in which a stimulus is detected by a receptor, processed by a control centre, acted on by an effector, and the response opposes the original change.