Negative and Positive Feedback

Homeostatic control relies on feedback: the output of a system is fed back to its input so that the system can correct itself. In mammals, feedback comes in two contrasting forms. Negative feedback reverses a change and is the dominant mechanism of homeostasis, keeping variables like body temperature and blood glucose close to a set point. Positive feedback amplifies a change and drives a process to completion — it is far less common, but plays crucial roles in childbirth, blood clotting and the generation of action potentials. This lesson explains both systems in the detail required for OCR A-Level Biology A specification module 5.1.1(d)–(e).

Key Definitions:

• Negative feedback — a control mechanism in which a change in a variable triggers a response that reverses the change, restoring the variable to its set point.
• Positive feedback — a control mechanism in which a change in a variable triggers a response that amplifies the change, moving the variable further from its starting value.
• Set point — the normal or target value of a controlled variable.

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The Generic Feedback Loop

Every feedback loop in a mammal has the same basic architecture: receptors detect a change; a coordinator (often a region of the brain or an endocrine gland) interprets the signal; effectors carry out the response; and the result is detected again by the receptors, closing the loop.

flowchart LR
    A[Stimulus: change in variable] --> B[Receptor]
    B --> C[Coordinator / central control]
    C --> D[Effector]
    D --> E[Response]
    E -.feedback.-> A

If the response reverses the stimulus, the feedback is negative. If the response reinforces the stimulus, it is positive.

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Negative Feedback

Negative feedback is the basis of almost all homeostatic control in mammals. It keeps variables oscillating gently around a set point rather than drifting uncontrollably.

Example 1 — Temperature Regulation

1. Core body temperature rises above 37 °C.
2. Thermoreceptors in the hypothalamus detect the rise.
3. The hypothalamus sends impulses to effectors: sweat glands begin to secrete, and arterioles in the skin dilate.
4. Evaporation of sweat and increased heat loss cool the body.
5. The thermoreceptors detect that core temperature is returning to normal and reduce their signalling — the response switches off.

Example 2 — Blood Glucose Regulation

1. Blood glucose rises after a meal.
2. β cells in the pancreatic islets of Langerhans detect the rise.
3. They release insulin into the bloodstream.
4. Insulin causes liver and muscle cells to take up glucose and store it as glycogen.
5. Blood glucose falls; insulin secretion decreases.

Features of a Negative Feedback System

• The response is always opposite in direction to the change.
• It is self-limiting — as the variable returns to normal, the response weakens and then stops.
• It produces oscillation around a set point. No feedback system responds instantaneously, so the variable typically overshoots slightly, is corrected, overshoots in the other direction, and so on, settling into small oscillations around the set point.
• Multiple negative feedback loops often work simultaneously (e.g., insulin and glucagon together regulate blood glucose).

Set Points and Ranges

The set point is not a single fixed number but a narrow range around which the variable oscillates. For human core temperature, the set point is ~37 °C with daily fluctuations of ±0.5 °C. For blood glucose, fasting levels sit around 4–6 mmol dm⁻³ and rise to around 7–8 mmol dm⁻³ after a meal.

flowchart TB
    A[Rising variable] --> B[Receptor detects increase]
    B --> C[Coordinator signals effector]
    C --> D[Effector reduces variable]
    D --> E[Variable returns to set point]
    F[Falling variable] --> G[Receptor detects decrease]
    G --> H[Coordinator signals opposing effector]
    H --> I[Effector increases variable]
    I --> E

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Positive Feedback

Positive feedback is rare in mammals because it is inherently destabilising — a small change is amplified until the system reaches a new end-point. It is used when rapid, all-or-nothing responses are required.

Example 1 — Oxytocin and Childbirth