Homeostasis

Homeostasis describes any process by which the body maintains variable concentrations of a substance in the body within a narrow range. This is usually accomplished by sets of positive and negative feedback loops, many of which are tied to hormones which are triggered or suppressed by certain substances in the body. As such, the body can maintain equilibrium in varied circumstances, ranging from hunger to having just finished a full meal, and from sleep to intense exertion. The nine key such systems in humans are:
#Maintaining core body temperature, through mechanisms such as sweat
#Maintaining constant blood sugar levels, through the use of insulin and glucagon
#Maintaining calcium levels in plasma through the use of the kidneys and skeleton
#Maintaining blood gases through regulation of breathing
#Maintaining arterial blood pressure, through the heart and kidneys
#Maintaining the level of sodium outside of cells through the kidneys
#Maintaining the level of potassium outside of cells through the kidneys
#Maintaining the body's levels of water through the kidneys and by inducing thirst
#Maintaining the PH of extracellular fluid
Several diseases are the manifestation of the breakdown of one or more homeostasis systems, such as diabetes mellitus.
Homeostasis at Wikipedia