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Homeostasis 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 why 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.
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
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
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