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 reqires 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
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
