14.1.1—Homeostasis definition and importance

Homeostasis is the regulation of internal conditions so that a cell or organism maintains conditions suitable for normal function, despite internal or external change. A regulated variable can change, but it is kept within a functional range rather than allowed to drift without control.

Internal variables such as temperature, water potential, pH and dissolved respiratory gases affect enzyme action, membranes and cell function. Keeping them within workable limits protects the reactions and physical conditions on which cells depend.

A generic negative-feedback framework:

1. A receptor or sensor detects that a regulated variable has deviated from its acceptable range.
2. A coordination centre receives or processes the information and selects a corrective response.
3. An effector—such as a muscle or gland—changes its activity.
4. The response opposes the original deviation, so the variable moves back towards its functional range.
5. Monitoring continues; the response is reduced when the deviation is corrected.

Negative feedback does not mean the variable never changes, and homeostasis is not equilibrium or a permanently fixed value. This card gives the general control architecture; particular organs, hormones and named feedback examples belong to later cards.