Homeostasis is the maintenance of constancy of the internal environment of the body.
Claude Bernard introduced the concept of milieu intérieur; Walter Bradford Cannon coined the term "homeostasis" in 1929.
Feedback Mechanisms
Homeostatic regulation operates through feedback systems that safeguard a set point already fixed for a physiological variable.
A change in the variable is detected by a sensor, which activates the feedback system that triggers a response through an effector to restore the variable to its set point.
Negative feedback is the usual homeostatic mechanism: a rise in the variable above set point triggers processes that inhibit its further formation, and a fall below set point withdraws that inhibition.
Components: the sensor (receptors monitoring the variable, e.g. baroreceptors in carotid sinus and aortic arch) and the control centre (usually CNS, e.g. medulla and hypothalamus for blood pressure).
The effector is the target organ producing the response, e.g. heart and blood vessels for blood pressure regulation.
Example: a rise in blood pressure increases baroreceptor discharge to the medullary centre, which inhibits sympathetic drive to heart and vessels until pressure returns to the set point.
Gain of a feedback system = Correction ÷ Error; a baroreceptor gain of 2 means a disturbance raises blood pressure only one-third as much as it would without the reflex.
Gain for temperature regulation can be as high as 30.
Positive feedback: a rise in the variable triggers processes that further increase it, a self-amplifying cycle that terminates only when the trigger is withdrawn or the process self-terminates; it does not provide homeostasis.