Homeostasis and negative feedback - TCE Biology (Tasmania)
“Homeostasis involves a stimulus-response model in which a change in external or internal environmental conditions is detected and appropriate responses occur via negative feedback; in vertebrates, receptors and effectors are linked via a control centre by nervous or hormonal pathways, or both”
Homeostasis is keeping the internal environment (such as body temperature, blood glucose and water balance) within narrow tolerance limits despite changes inside or outside the body. It follows a stimulus-response model: a stimulus (a change) is detected by a receptor, a control centre processes it, and an effector produces a response that counteracts the change. This is negative feedback, because the response reduces the original stimulus. In vertebrates, receptors and effectors are linked by nervous pathways, hormonal pathways, or both.
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What this dot point is asking
Homeostasis is criterion 6 (Section C of the TASC exam). Every question on thermoregulation, glucose regulation or osmoregulation uses the model on this page, so learn it as a template.
The stimulus-response model
| Part | Meaning | Example (blood glucose rises after a meal) |
|---|---|---|
| Stimulus | A change in the internal or external environment | Blood glucose rises above the normal range |
| Receptor | Detects the change | The pancreas detects the rise |
| Control centre | Processes the information and triggers a response | The pancreas releases insulin |
| Effector | Carries out the response | Liver and body cells |
| Response | The action that counteracts the change | Cells take up glucose; the liver stores it as glycogen |
The result is that the variable returns towards its set point, which reduces the stimulus.
Negative feedback
In negative feedback, the response reduces or reverses the original change. As the variable returns to normal, the stimulus weakens, so the response is switched down. This self-regulating loop keeps the variable fluctuating within narrow limits around a set point rather than holding it perfectly constant.
The response acts in the opposite direction to the stimulus: a rise produces responses that cause a fall, and a fall produces responses that cause a rise.
Tolerance limits
Every organism can survive only within a range of conditions, its tolerance limits. Homeostatic mechanisms, along with structural features and behaviour, keep internal conditions within those limits. If a variable moves too far outside them (severe dehydration, very high body temperature), enzymes and cells stop working properly.
Nervous and hormonal pathways
| Nervous pathway | Hormonal pathway | |
|---|---|---|
| Signal | Electrical impulses along neurons; neurotransmitters at synapses | Hormones travelling in the blood |
| Speed | Very fast | Slower |
| Duration | Short-lived | Often longer lasting |
| Target | Specific cells connected by neurons | Any cell with the matching receptor |
| Example | Shivering in the cold | Insulin lowering blood glucose |
Many homeostatic responses use both. See nerve impulses and synaptic transmission and hormones and the endocrine system.
A template for any variable
Draw a loop of boxes connected by arrows:
Stimulus (variable rises) → Receptor (detects the rise) → Control centre (processes, sends a signal) → Effector (acts) → Response (variable falls) → back to normal, which reduces the stimulus.
Draw a second loop below for a fall in the variable. Label the pathway (nervous or hormonal) on the arrow from the control centre to the effector.
Marker's note: TASC markers accept clearly labelled flow diagrams; name the actual structures and hormones, not just "receptor" and "effector".
- Naming an organ without its role
- Say which structure is the receptor, the control centre and the effector.
- Mixing up the variables
- The hypothalamus is the control centre for temperature and water balance; the pancreas is the receptor and control centre for blood glucose. TASC's 2025 assessment report noted that many lower-scoring students named the hypothalamus, pituitary or kidneys as receptors or control centres in a blood glucose answer.
- Describing positive feedback
- Homeostasis relies on negative feedback: the response reduces the stimulus.
Exam-style questions
Questions in the style of TASC 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.
Original5 marksUse the stimulus-response model to explain how negative feedback returns body temperature to normal after a person starts running on a hot day.Show worked answer →
One mark for each correctly applied element.
- Stimulus
- Body temperature rises above the normal range because of heat from muscle activity and the hot environment.
- Receptor
- Thermoreceptors in the skin and in the hypothalamus detect the rise.
- Control centre
- The hypothalamus processes the information and sends nerve impulses to effectors.
- Effectors and response
- Sweat glands increase sweat production and skin arterioles dilate (vasodilation), increasing heat loss.
- Negative feedback
- As body temperature falls back towards normal, the stimulus is reduced, so the receptors detect less change and the response decreases.
Original4 marksCompare nervous and hormonal pathways as ways of linking receptors to effectors.Show worked answer →
Two marks for each point of comparison (any two).
- Signal
- Nervous: electrical impulses along neurons and chemical neurotransmitters at synapses. Hormonal: chemical hormones carried in the blood.
- Speed
- Nervous responses are very fast (milliseconds); hormonal responses are slower (seconds to hours).
- Duration
- Nervous responses are usually short-lived; hormonal responses often last longer.
- Target
- Nervous signals go to specific cells along a direct pathway; hormones reach all cells through the blood but only affect target cells with the matching receptors.