Homeostasis

Homeostasis is the process by which a living organism keeps its internal environment relatively stable. Body temperature, blood glucose, blood pressure, water balance and the pH of body fluids each have a set point, the value they hover around, and a normal range within which cells work well. The balance is dynamic: values drift all the time and are steadily pushed back. The American physiologist Walter Cannon introduced the term in the 1920s.

Most homeostasis relies on negative feedback, which reverses a change. A sensor detects the variable, a control center, often in the brain, compares it with the set point, and an effector makes the correction. When the body overheats, skin blood vessels widen and sweating increases; when it cools, blood flow to the skin drops and shivering begins. After a meal, rising blood glucose triggers insulin release, which brings glucose back down. Positive feedback, which amplifies a change, is kept for events with a clear end point, such as childbirth and blood clotting.

Many diseases can be understood as a failure of homeostasis. In diabetes mellitus, for example, blood glucose is no longer held within its normal range.