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.

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.

Examples in context

Example 1. Cairns dengue fever and thermoregulatory set point. Cairns Hospital admits dengue patients during the wet season when Aedes aegypti mosquito numbers peak. Dengue virus and the immune response release pyrogens (interleukin-1, prostaglandin E2) that act on the hypothalamus to raise the temperature set point from 37.0 to around 39.0 degrees Celsius. The new set point drives shivering (effector: skeletal muscle), vasoconstriction (effector: arterioles) and behavioural seeking of warmth, until core temperature reaches the elevated set point. Once the infection clears, the set point falls back and the same circuits drive sweating and vasodilation. The case shows the stimulus, receptor (hypothalamic neurons), control centre (hypothalamus), effector and response framework.

Example 2. QIMR Berghofer blood pressure baroreflex. Researchers at QIMR Berghofer studying cardiovascular risk in Indigenous Queensland communities monitor the baroreflex. A rise in blood pressure stretches the carotid sinus and aortic arch baroreceptors (sensors), action potentials reach the medulla oblongata (control centre), which reduces sympathetic and increases parasympathetic outflow to the sinoatrial node (effector). Heart rate falls within seconds; cardiac output and blood pressure return toward 120/80 mmHg. The opposite occurs when blood pressure drops during haemorrhage. This rapid loop exemplifies negative feedback returning a parameter to a set point.

Try this

Q1. Define homeostasis and identify the five components of a negative-feedback loop using body temperature as the example. [4 marks]

• Cue. Stable internal environment despite external change. Stimulus (heat), receptor (skin), control (hypothalamus), effector (sweat gland), response (cooling).

Q2. A continuous blood-glucose monitor on a type 2 diabetic shows readings 5.5, 5.8, 6.2, 9.0, 11.0, 8.5, 6.8 and 5.6 mmol/L across two hours after lunch. Identify the homeostatic disturbance and explain why the rise is slower to correct than in a healthy person. [3 marks]

• Cue. Hyperglycaemia. Insulin resistance reduces effector responsiveness; correction takes longer.

Q3. Compare negative and positive feedback. (a) Define each. (b) Give one example of positive feedback in childbirth. (c) Justify why most homeostatic mechanisms are negative rather than positive. [2+2+2 marks]

• Cue. (a) Negative reverses change; positive amplifies. (b) Oxytocin and uterine contractions. (c) Positive feedback is destabilising; needed only for finite events.