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4.4 Feedback

Syllabus
2025
Topic
4.4
Level
—

Learning objectives

4.4A—Explain how positive and negative feedback helps maintain homeostasisExplain how positive and negative feedback helps maintain homeostasis.• Organisms use feedback mechanisms to maintain their internal environments in response to internal and external changes.- i. Negative feedback mechanisms maintain homeostasis by reducing the initial stimulus to regulate physiological processes. If a system is perturbed or disrupted, negative feedback mechanisms return the system back to its target set point. These processes operate at the molecular, cellular, and organismal levels.- ii. Positive feedback mechanisms amplify responses and processes in biological organisms. The variable initiating the response is moved further away from the initial set point. Amplification occurs when the stimulus is further intensified, which, in turn, initiates an additional response that produces system change.

How Negative and Positive Feedback Regulate Biological Systems

A feedback mechanism detects a change in a biological variable and produces a response that feeds back on the original stimulus. The direction of that response distinguishes negative feedback from positive feedback.

Feedback type Effect on the initial change System outcome
Negative feedback Reduces or reverses the change The variable moves back toward its target set point, helping maintain dynamic homeostasis
Positive feedback Amplifies the change The variable moves farther from its starting point until a defined endpoint or outside event stops the loop

Negative-feedback example: when blood glucose rises, signals promote glucose uptake and storage, so blood glucose falls toward its set point. Positive-feedback example: platelets at a damaged blood vessel release signals that recruit more platelets, strengthening the response until the break is sealed.

To classify a loop, identify the initial change and ask what the response does to it. A response that opposes the change is negative feedback; a response that intensifies the same change is positive feedback.

Positive does not mean beneficial and negative does not mean harmful. The terms describe the direction of feedback. Positive feedback usually drives a process to completion rather than restoring a set point directly.