From Wikipedia,
the free encyclopedia.
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 the
Greek roots homo-
(same, like) and sta- (to
stand or stay).
Overview
The term is most often used in
the sense of
biological homeostasis. homeo-
similar or same. stasis- standing
or stopping.
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.
Complex systems, such as a
human body, must have homeostasis
to maintain stability and to
survive. These systems do not only
have to endure to survive; they
must adapt themselves and evolve
to modifications of the
environment.
Properties of homeostasis
Homeostatic systems show
several properties:
- They are
ultrastable;
- Their whole
organization, 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).
Main examples of homeostasis in
mammals are as follows:
- The regulation of the
amounts of water and minerals in
the body. This is known as
osmoregulation. This happens in
the kidneys.
- The removal of metabolic
waste. This is known as
excretion. This is done by the
excretory organs such as the
kidneys and lungs.
- The regulation of body
temperature. This is mainly done
by the skin.
- The regulation of blood
glucose level. This is mainly
done by the liver and the
insulin secreted by the
pancreas.
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.
Thermoregulation is another
example of negative feedback.
When body temp rises (or falls),
receptors in the skin and the
hypothalamus sense a change,
triggering a command from the
brain, which in turn effects the
correct response, this case
being body temp decreases.
- 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.)
Blood clotting and events in
childbirth are other types
of positive feedback.
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. When sunlight is
plentiful and atmospheric
temperature climbs, the
phytoplankton of the ocean
surface waters thrive and produce
more
dimethyl sulfide, DMS. The DMS
molecules act as
cloud condensation nuclei
which produce more clouds and thus
increase the atmospheric
albedo and lower the
temperature of 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.
The internal environment of a
living organism's body features
body fluids in multicellular
animals. The body fluids include
blood plasma,
tissue fluid and
intracellular fluid. The
maintenance of a
steady state in these fluids
is essential to living things as
the lack of it harms the genetic
material.
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 are able to eat just
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, and
acidity, and the
concentrations of nutrients such
as
glucose, various
ions,
oxygen, and wastes, such as
carbon dioxide and
urea. 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.
Other fields
The term has come to be used in
other fields, as well.
An
actuary may refer to "risk
homeostasis", where (for example)
people who have anti-lock brakes
have no better safety record than
those without anti-lock brakes,
because they unconsciously
compensate for the safer vehicle
via less-safe driving habits.
Sociologists and psychologists
may refer to "stress homeostasis",
the tendency of a population or an
individual to stay at a certain
level of stress, often generating
artificial stresses if the
"natural" level of stress is not
enough.
Examples
Most of these organs are
controlled by
hormones secreted from the
pituitary gland, which in turn
is directed by the
hypothalamus.
See also
|
Edit |
General subfields and
scientists in
Cybernetics |
|
K1 |
Ergodic theory,
Polycontexturality,
Second order cybernetics |
|
K2 |
Catastrophe theory,
Connectionism,
Control theory,
Decision theory,
Game theory,
Information theory,
Semiotics,
Synergetics,
Systems theory |
|
K3 |
Biological cybernetics,
Biomedical cybernetics,
Biorobotics,
Computational neuroscience,
Homeostasis,
Medical cybernetics,
Neuro cybernetics,
Sociocybernetics |
|
Cyberneticians |
William Ross Ashby,
Claude Bernard,
Valentin Braitenberg,
Ludwig von Bertalanffy,
George S. Chandy,
Joseph J. DiStefano III,
Heinz von Foerster,
Charles François,
Jay Forrester,
Ernst von Glasersfeld,
Francis Heylighen,
Erich von Holst,
Stuart Kauffman,
Niklas Luhmann,
Warren McCulloch,
Humberto Maturana,
Horst Mittelstaedt,
Talcott Parsons,
Walter Pitts,
Alfred Radcliffe-Brown,
Robert Trappl,
Valentin Turchin,
Francisco Varela,
Frederic Vester,
John N. Warfield,
Kevin Warwick,
Norbert Wiener |
