Homeostasis or homoeostasis is the property of a system in which a variable (for example, the concentration of a substance in solution, or its temperature) is actively regulated to remain very nearly constant. This regulation occurs inside a defined environment (mostly within a living organism's body). Examples of homeostasis include the regulation of the body temperature of an animal, the pH of its extracellular fluids, or the concentrations of sodium (Na+) and calcium (Ca2+) ions or of glucose in the blood plasma, despite changes in the animal’s environment, or what it has eaten, or what it is doing (for example, resting or exercising). Each of these variables (for example, body temperature, the pH, or the Na+, Ca2+ and glucose concentrations) is controlled by a separate “homeostat” (or regulator), which, together, maintain life. Homeostats are energy-consuming physiological mechanisms.[1]
Although the term was originally used to refer to processes within living organisms, it is frequently applied to technological control systems such as thermostats. A homeostat has an absolute requirement for a sensor to detect changes in the controlled entity's value, as well as an effector mechanism that reverses any detected deviation from the desired value (or “setpoint”) of the regulated entity. Since the correction of any error detected by the sensor is always in the opposite direction to the error, a homeostat relies on what is known as a negative feedback connection between the sensor and effector.[3][4] The effector's corrective effects are monitored by the sensor, which turns the corrective measures off when setpoint conditions have been restored.[5] Negative feedback systems are therefore referred to as "closed loop", or "negative feedback loops", to distinguish them from "open loop" systems where a stimulus (acting on a sensor) results in an, often, all-or-none response that is not subject to modification once it has been set in motion.[3][6][7]
