Homeostasis

Homeostasis or homoeostasis is the property of an open system to regulate its internal environment so as to maintain a stable condition, by means of multiple dynamic equilibrium adjustments controlled by interrelated regulation mechanisms.

Multicellular organisms require a homeostatic internal environment, in order to live; many environmentalists believe this principle also applies to the external environment.

Properties of homeostasis

Homeostatic systems show several properties:

• They are ultrastable;
• Their whole organisation, internal, structural, and functional, contributes to the maintenance of equilibrium
• They are unpredictable (the resulting effect of a precise action often has the opposite effect to what was expected).

Mechanisms of homeostasis: feedback

Main article: feedback

When a change of variable occurs, there are two main types of feedback to which the system reacts:

• Negative feedback is a reaction in which the system responds in such a way as to reverse the direction of change. Since this tends to keep things constant, it allows the maintenance of homeostasis. For instance, when the concentration of carbon dioxide in the human body increases, the lungs are signalled to increase their activity and expel more carbon dioxide.
• In positive feedback, the response is to amplify the change in the variable. This has a de-stabilizing effect, so does not result in homeostasis. Positive feedback is less common in naturally occurring systems than negative feedback, but it has its applications. For example, in nerves, a threshold electric potential triggers the generation of a much larger action potential. (See also leverage points.)

Ecological homeostasis

In the Gaia hypothesis, James Lovelock stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast organism that actively modifies its planet to produce the environment that suits its needs. In this view, the entire planet maintains homeostasis. Whether this sort of system is present on Earth is still open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, plants are able to grow better and thus remove more carbon dioxide from the atmosphere.

Biological homeostasis

Homeostasis is one of the fundamental characteristics of living things. It is the maintenance of the internal environment within tolerable limits.

With regard to any parameter, an organism may be a conformer or a regulator. Regulators try to maintain the parameter at a constant level, regardless of what is happening in its environment. Conformers allow the environment to determine the parameter. For instance, endothermic animals maintain a constant body temperature, while ectothermic animals exhibit wide variation in body temperature.

This is not to say that conformers may not have behavioral adaptations that allow them to exert some control over the parameter in question. For instance, reptiles often sit on sun-heated rocks in the morning to raise their body temperatures.

An advantage of homeostatic regulation is that it allows the organism to function more effectively. For instance, ectotherms tend to become sluggish at low temperatures, whereas endotherms are as active as always. On the other hand, regulation requires energy. One reason snakes can eat only once a week is that they use much less energy for maintaining homeostasis.

Homeostasis in the human body

All sorts of factors affect the suitability of the human body fluids to sustain life; these include properties like temperature, salinity, acidity (carbon dioxide), and the concentrations of nutrients and wastes (urea, glucose, various ion, oxygen). Since these properties affect the chemical reactions that keep bodies alive, there are built-in physiological mechanisms to maintain them at desirable levels.

However, it should be noted that homeostasis is not the reason for these ongoing unconscious adjustments. Homeostasis should be thought of as a general characterization of many normal processes in concert, not their proximal cause per se. Moreover, there are numerous biological phenomena which do not conform to this model, such as anabolism.

Examples

• Thermal regulation:
  • The skeletal muscles can shiver to produce heat if the body temperature is too low.
  • Non-shivering thermogenesis involves the decomposition of fat to produce heat.
  • Sweating cools the body with the use of evaporation.
• Chemical regulation
  • The pancreas produces insulin and glucagon to control blood-sugar concentration.
  • The lungs take in oxygen and give off carbon dioxide.
  • The kidneys remove urea, and adjust the concentrations of water and a wide variety of ions.

Most of these organs are controlled by hormones secreted from the pituitary gland, which in turn is directed by the hypothalamus.

Etymology

The term was coined in 1932 by Walter Cannon from two Greek words (to remain the same).

See also

• Acclimatization
• Biological rhythm
• Metabolism
• Apoptosis
• Aging
• Balance
• Self-organization
• Cybernetics