14.1.2—Homeostasis control principles

Negative feedback is a control loop in which a deviation in a regulated variable triggers a response that opposes the original change. The aim is to return the variable towards its functional range, not to prevent all change.

Ordered control chain:

1. A stimulus moves a regulated variable away from its target or acceptable range.
2. A receptor detects the deviation and sends information to an integrating or coordination centre.
3. The coordinator compares the information with the required range and sends a signal to a suitable effector.
4. The effector changes its activity in the direction that opposes the original deviation.
5. As the variable returns towards its functional range, the corrective signal is reduced; monitoring then continues.

Generic read-off: if a variable rises above its functional range, the response should act to lower it; if it falls below the range, the response should act to raise it. The same receptor→coordinator→effector architecture can use nervous signals, hormones or both, depending on the control system.

“Negative” describes the response direction relative to the initial deviation; it does not mean harmful or ineffective. A response that amplifies the original change is positive feedback, not negative feedback. Specific organ, hormone and named physiological examples belong to later cards.