Physiology
Homeostasis and negative feedback
Homeostasis is the organizing principle of physiology — once you can see the loop, most regulatory systems in the body follow the same shape.
The components of a feedback loop
Every homeostatic system has the same four parts, and exam answers earn marks for naming them explicitly:
- Stimulus — a change in the variable being regulated
- Receptor / sensor — detects the change
- Control centre — compares the value against a set point (often the hypothalamus)
- Effector — produces the response that opposes the change
Negative feedback
In negative feedback the response opposes the original stimulus, returning the variable toward its set point. It's the mechanism behind blood glucose control, thermoregulation, blood pressure, blood pH, and osmoregulation. The variable oscillates gently around the set point rather than sitting perfectly still.
Worked example: thermoregulation
| Too hot | Too cold | |
|---|---|---|
| Blood vessels | Vasodilation — more heat lost at skin | Vasoconstriction — heat conserved |
| Sweat glands | Increased sweating, evaporative cooling | Sweating stops |
| Muscle | — | Shivering generates heat |
| Metabolic rate | — | Increases (thyroid, adrenaline) |
Positive feedback
Positive feedback amplifies the stimulus rather than opposing it, driving the system further from its starting point. It is much rarer, and used where a process needs to run rapidly to completion:
- Blood clotting — activated platelets recruit more platelets
- Childbirth — oxytocin strengthens contractions, which triggers more oxytocin
- Action potentials — sodium influx opens further sodium channels
Frequently confused
"Positive" and "negative" describe the direction of the response relative to the stimulus, not whether the outcome is good or bad. Positive feedback is not "beneficial feedback" — uncontrolled positive feedback is often dangerous.
Frequently asked questions
What are the four components of a feedback loop?
A stimulus, a receptor that detects the change, a control centre that compares the value against a set point, and an effector that produces the response.
What is the difference between negative and positive feedback?
Negative feedback opposes the original stimulus and returns a variable toward its set point. Positive feedback amplifies the stimulus, driving the system further from its starting point, as in blood clotting and childbirth.
Is positive feedback harmful?
Not inherently. The terms describe the direction of the response relative to the stimulus, not whether the outcome is desirable. Positive feedback is used where a process must run quickly to completion, though uncontrolled positive feedback can be dangerous.
Keep reading: Insulin and glucagon · Kidney function explained · Electrolytes explained