The main focus of physiology is the understanding of the mechanisms by which cells, organs, and organ systems maintain homeostasis. This is accomplished primarily by negative feedback. The general concept is that the body tries to increase a variable when it is below its optimum (termed the set point), and decrease a variable when it is above its optimum.

This is analogous to the thermostat that controls room temperature by adjusting the heating and/or celling of the room. For example, if you open a window on a cold day, the room temperature decreases from the set point of the thermostat. This is called perturbation. The thermostat contain a sensor that detects the difference between the room temperature and the set point.

The thermostat signals the furnace to generate heat, and the room temperature is returned toward normal. The difference between the low point in room temperature and the final room temperature at steady state is called the correction.

Because the window is left open in this example, room temperature does not quite return to the set point; the remaining difference between the final room temperature and its thermostatic set point is called the error.

The ability of the control system to restore the system to its set point is called gain.

Gain = Correction/Remaining error