Physiology · Cell and membrane
Homeostasis and feedback control !
- Concept
- Exam priority: high
The body measures its variables (temperature, glucose, pressure) and corrects them when they drift from a target value.
Updated 2026-11-01
Also called: Negative feedback control, Positive feedback
What it is
Homeostasis is the body's ability to keep its internal environment (the extracellular fluid) stable despite outside change. Nearly every physiological control works as a loop: a sensor detects the variable, a centre compares it with a set point and an effector corrects it. Negative feedback stabilises; positive feedback amplifies and is useful only when it has a defined end.
Key facts
- The internal environment being regulated is the extracellular fluid: cells take oxygen, nutrients and ions from it.
- Fast controls are neural (seconds); slow, sustained ones are hormonal (minutes to days).
- Feedforward control acts before the error: the cephalic phase of digestion, or ventilation rising as exercise starts.
- A vicious circle is positive feedback without a brake: in severe shock, falling cardiac output cuts coronary flow, which weakens the heart further.
How it works, step by step
- A variable drifts from its set point (e.g. blood pressure falls on standing up).
- A sensor detects it (carotid sinus and aortic arch baroreceptors) and fires less.
- The integrating centre (medulla) compares the signal with the set point and computes the error.
- Effectors respond: heart rate, contractility and vascular tone rise.
- Pressure returns towards the set point; the error shrinks and the response switches off.
Equations
- correction
- how much of the change the system cancels
- remaining error
- how much of the change persists
Our example: if a disturbance would move a variable 30 units and the control leaves it at 3, the correction is 27 and the gain 9.
Types of control
| Type | What it does to the change | Examples |
|---|---|---|
| Negative feedback | Opposes it and stabilises | Thermoregulation, blood glucose, baroreflex, thyroid axis |
| Positive feedback | Amplifies it until an end event | Labour (oxytocin), clotting, LH surge, Na⁺ depolarisation |
| Feedforward | Acts before the error appears | Cephalic phase of digestion, response at the start of exercise |
Clinical notes
- Shock: if compensation falls short, hypoperfusion damages the heart and vessels and pressure falls further (vicious circle).
- Diabetes mellitus: the effector (insulin) or the response to it fails; blood glucose no longer returns to its set point.
- Endocrine tests use negative feedback: a high peripheral hormone should suppress its stimulating hormone.
Test yourself
Think of the answer, then tap the question to see it.
Why does the positive feedback of labour not become a dangerous vicious circle?
It has an end event: once the baby is delivered, cervical stretch stops, oxytocin release falls and the loop switches off.
A control reduces a 20-unit change to 2. What is its gain and what does it mean?
It corrects 18 and leaves 2 as error: a gain of 9. Higher gain means less remaining error, but no negative control removes it completely.
In fever, why does the patient shiver and feel cold at the start?
The hypothalamus raises the set point; actual temperature sits below it and is sensed as an error, so the body makes heat (shivering) and conserves it (vasoconstriction).
How does feedforward control differ from negative feedback?
Feedback waits for an error and corrects it; feedforward predicts the change and acts first, like gastric secretion at the smell of food.
Related topics
- Body temperature regulation
- The baroreceptor reflex
- Hormones: types, receptors and feedback
- Parturition: onset and uterine contraction
- Fever
- The hypothalamic–pituitary system: overview
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: 1.5 Homeostasis (OpenStax (Rice University), 2022); 15.2 Autonomic Reflexes and Homeostasis (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 1). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.