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# 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

## Key points

- Negative feedback: the response opposes the change (temperature, blood glucose, blood pressure). It is the body's dominant control.
- Positive feedback: the response reinforces the change; useful when an outside event ends it (labour, clotting, the LH surge, Na⁺ channel opening in the action potential).
- The five parts of the loop: variable, sensor, integrating centre with a set point, effector and the error signal.
- Gain = correction / remaining error: the higher it is, the tighter the control. No negative system corrects 100%.
- Fever is not a failure of homeostasis: it is a raised set point that the body actively defends.

## 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

**Gain of a control system**Gain = correction / remaining error

**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.

## Memory aid

Negative neutralises, positive pushes: negative feedback calms a change; positive pushes it on until something stops it.

## 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.

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## Related topics

- [Body temperature regulation](https://durgical.com/en/physiology/body-temperature-regulation/)
- [The baroreceptor reflex](https://durgical.com/en/physiology/the-baroreceptor-reflex/)
- [Hormones: types, receptors and feedback](https://durgical.com/en/physiology/hormones-types-receptors-and-feedback/)
- [Parturition: onset and uterine contraction](https://durgical.com/en/physiology/parturition-onset-and-uterine-contraction/)
- [Fever](https://durgical.com/en/physiology/fever/)
- [The hypothalamic–pituitary system: overview](https://durgical.com/en/physiology/the-hypothalamic-pituitary-system-overview/)

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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](https://openstax.org/books/anatomy-and-physiology-2e/pages/1-5-homeostasis) (OpenStax (Rice University), 2022); [15.2 Autonomic Reflexes and Homeostasis](https://openstax.org/books/anatomy-and-physiology-2e/pages/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.

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