The big idea: Homeostasis is keeping the body's internal environment roughly constant, even when the outside world changes.
Each regulated factor has a set point — the normal value the body aims for (for example, core temperature near 37 °C).
The body does this mainly by negative feedback: whenever a factor drifts away from its set point, the body produces a response that pushes it back — that one idea is what the exam tests.
- Homeostasis
- Keeping the internal environment of the body within narrow limits, close to a constant set point, despite changes outside.
- Internal environment
- The conditions inside the body — the blood and tissue fluid — such as temperature, glucose, pH and water content.
- Set point
- The normal value a regulated factor is held close to (e.g. ~37 °C for core temperature, ~pH 7.4 for blood).
- Negative feedback
- A control loop in which the response OPPOSES the change, returning the variable toward its set point. It is the basis of homeostasis.
Blood variables the body regulates: The exam usually frames homeostasis around blood variables kept near a set point:
Blood glucose concentration.
Core (blood) temperature.
Blood pH (set by carbon-dioxide level).
Water / solute balance of the blood.
Each is held steady by the same kind of negative-feedback loop — only the receptors and effectors differ.
Every homeostatic control follows the same four-stage loop. Read it as a chain of cause and effect: a change happens, it is detected, it is processed, and a response is produced that cancels the change.
The key feature is the last step — the response acts in the opposite direction to the change, which is exactly why it is called negative feedback.
The loop, step by step
- A stimulus (some change) shifts a variable away from its set point.
- A receptor detects the change in the internal environment.
- A control centre (the processor — often the hypothalamus) compares the value with the set point and decides on a response.
- It signals an effector (a muscle or a gland) to act.
- The effector produces a response that OPPOSES the change — a rise triggers a fall, a fall triggers a rise.
- The variable returns toward its set point, and the loop switches off until the next change.
The negative-feedback loop: a change shifts a blood variable away from its set point, receptors detect it, the control centre signals effectors, and the response OPPOSES the change — pulling the variable back to normal.
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Why 'opposing' is the whole trick: Negative feedback works because the response is always opposite to the change.
If a variable rises too high, the loop triggers a response that makes it fall.
If it falls too low, the loop triggers a response that makes it rise.
So the variable can never run away — it is constantly nudged back. It fluctuates around the set point rather than sitting exactly on it, but it is always held within narrow limits.
| Stage of the loop | What it does | Example: body too warm |
|---|---|---|
| Stimulus (the change) | A factor pushes the variable away from its set point | Core temperature rises above ~37 °C |
| Receptor | Detects the change in the internal environment | Heat-sensitive receptors in the skin and hypothalamus |
| Control centre (processor) | Compares the value with the set point and signals the effectors | The hypothalamus in the brain |
| Effector | Carries out the response (muscle or gland) | Sweat glands, skin arterioles |
| Response | OPPOSES the change, returning the variable toward normal | Sweating + vasodilation cool the body down |
The same loop, three blood variables: The structure never changes — only the parts:
Glucose too high → pancreas releases insulin → cells take up glucose → glucose falls back to normal.
Body too warm → hypothalamus triggers sweating + vasodilation → heat is lost → temperature falls back to normal.
Blood pH falls (CO₂ high) → chemoreceptors trigger faster, deeper breathing → CO₂ is removed → pH rises back to normal.
Notice every example ends the same way: the change is reversed.
Read it as a chain: stimulus → receptor detects → control centre processes → effector responds → the response cancels the stimulus and the loop returns to the set point.
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How this is tested: A 4-mark Discuss on Paper 2 is the classic: discuss how positive and negative feedback are each used to control blood / body variables.
A Discuss needs both sides — describe negative feedback (response opposes the change → homeostasis) and positive feedback (response amplifies the change → drives a process to completion), with an example of each and a clear contrast.
Watch the trap: positive feedback is not homeostatic — saying it 'keeps the variable steady' loses the mark.
IB-style question — discuss positive and negative feedback
Discuss how negative feedback and positive feedback are each used to control variables in the body. [4]
How to score all four marks
- Negative feedback — the mechanism. The response opposes the change, returning the variable toward its set point (a rise triggers a fall, a fall triggers a rise).
- Negative feedback — its use. It is the basis of homeostasis: it keeps blood variables (e.g. glucose, temperature, pH) stable within narrow limits.
- Positive feedback — the mechanism. The response amplifies the change, driving the variable further from normal rather than back to it.
- Positive feedback — its use. It drives a process rapidly to completion then stops — e.g. the LH surge that triggers ovulation, or oxytocin in childbirth — so it is not homeostatic. (Award 1 mark for each distinct point, up to 4.)
Final answer
Negative feedback: the response opposes the change, returning a variable to its set point — this is homeostasis (e.g. blood glucose, temperature, pH). Positive feedback: the response amplifies the change, driving it further from normal to complete a process (e.g. the LH surge, oxytocin in childbirth) — so it is not homeostatic.
✓ Why this scores full marks: It discusses both — it does not just describe one. Each type gets a mechanism (what the response does to the change) and a use/example.
The contrast is explicit: opposes vs amplifies, homeostatic vs not. A Discuss that only covers negative feedback caps at about half marks.
| Negative feedback | Positive feedback | |
|---|---|---|
| Direction of the response | OPPOSES the change | AMPLIFIES the change |
| Effect on the variable | Returns it toward the set point | Drives it further from normal |
| Homeostatic? | Yes — it keeps the internal environment stable | No — it pushes a process to completion, then stops |
| Blood / body examples | Blood glucose, temperature, blood pH, water balance | The LH surge at ovulation; oxytocin in childbirth; blood clotting |