Homeostasis and negative feedback

Organisms (Continuity and change) · Homeostasis · note 1 of 7

Spec D3.3.1, D3.3.2Download all Homeostasis notes (PDF)Download all notes (PDF)11 pages

Homeostasis and negative feedbackSpec D3.3.1, D3.3.2

In short

Homeostasis is the maintenance of the internal environment of an organism: variables are kept within preset limits despite fluctuations in the external environment. In humans these include body temperature, blood pH, blood glucose concentration and blood osmotic concentration. Negative feedback returns a variable to its set point whether it rises above or falls below it.

Homeostasis is the maintenance of the internal environment of an organism. Variables are kept within preset limits, despite fluctuations in the external environment, so that cells, and especially enzymes, work reliably.

Homeostatic variables in humans
Variable	Approximate normal value
Body (core) temperature	About 37 °C
Blood pH	7.35–7.45
Blood glucose concentration	About 4–6 mmol L⁻¹ when fasting
Blood osmotic concentration	About 0.3 osmol L⁻¹

Negative feedback loops

A receptor detects a change in a variable away from its set point.
A control centre (often in the brain or an endocrine gland) compares it with the set point and sends signals by nerves or hormones.
Effectors (muscles or glands) respond.
The response reverses the change, returning the variable to the set point. The loop then switches off.

Negative feedback works in both directions: it brings a variable down when it is above the set point and up when it is below it.
Positive feedback amplifies a change away from the set point, so it would make an imbalance worse. That is why homeostasis uses negative feedback; positive feedback is only useful for events that must be completed quickly, such as childbirth.

Two negative feedback loops around a set point: when the variable rises, a receptor detects the change, the control centre sends signals and the effector responds so the variable falls; the mirror loop corrects a fall. A small graph shows the variable fluctuating around the set point between upper and lower limits. — Negative feedback corrects changes in both directions, so the variable fluctuates narrowly around the set point within preset limits.

Exam tip:

Negative feedback does not keep a variable perfectly constant: it fluctuates slightly around the set point within preset limits.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

Why does homeostasis use negative feedback rather than positive feedback?

Negative feedback reverses any change from the set point, so it returns a variable to normal whether it rises above or falls below it. Positive feedback amplifies a change away from the set point, which would make an imbalance worse. That is why positive feedback only controls processes that must finish quickly, such as childbirth.

How do insulin and glucagon control blood glucose?

When blood glucose rises, β cells in the pancreatic islets secrete insulin, which makes liver and muscle cells absorb glucose and store it as glycogen. When blood glucose falls, α cells secrete glucagon, which makes liver cells break glycogen down and release glucose. Both hormones travel to their target cells in the blood.

What is the difference between type 1 and type 2 diabetes?

In type 1 diabetes the immune system destroys the insulin-secreting β cells, so little or no insulin is made and insulin must be injected. In type 2 diabetes insulin is still produced but target cells become resistant to it. Type 2 is linked to obesity and inactivity and is managed mainly by diet and exercise.

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