Homeostasis or homoeostasis is the property of an open system, especially living organisms, to regulate its internal environment so as to maintain a stable condition, by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms.
The term was coined in 1932 by Walter Cannon from two Greek words (to remain the same).
Multicellular organisms require a homeostatic internal environment, in order to live; many environmentalists believe this principle also applies to the external environment. Many ecological, biological, and social systems are homeostatic. They oppose change to maintain equilibrium. If the system does not succeed in reestablishing its balance, it may ultimately lead the system to stop functioning.
The control of blood glucose levels is a good example of homeostasis. Most of the glucose utilized by the body is derived from the dietary intake of various forms of sugars and starches. These are digested within the intestinal tract into the simplest forms of carbohydrate (monosaccharides). Glucose, galactose, and fructose are the principal monosaccharides. These are absorbed from the intestines into the blood and enter the liver. Here all are eventually converted to glucose. The glucose may be utilized by the liver cells in part as a source of readily available energy, or it may be polymerized and stored as glycogen, but most of it enters the general circulation of the body and contributes to the blood glucose level. Blood glucose may also be derived in times of need by the conversion of the stored glycogen into glucose.
When food is eaten, there is a temporary rise in the blood glucose level known as alimentary hyperglycemia (high blood glucose level). Mechanisms are activated that stimulate the pancreas to secrete the hormone insulin. This hormone makes it possible for cells to utilize the glucose by facilitating its transport (carriage) across the membranes of cells into their interior, where it can enter the complex chemical reactions that provide the cell with energy. By virtue of insulin secretion, the cells receive adequate amounts of glucose, and the blood glucose levels are returned to the normal range, somewhere between 70 and 110 milligrams per 100 millilitres of blood.
Metabolic controls are exerted similarly for fats and proteins. As will be noted later on, derangements of these controls can lead to serious disease. The state of health implies proper, smooth-running metabolic machinery.
Main examples of homeostasis in mammals are as follows: