The term homeostasis
was used by physiologist Walter Cannon to refer to the
process by which an organism maintains a fairly constant internal (bodily)
environment. That is, how body temperature, blood-sugar level, salt concentration in
the blood, etc., are kept in a state of relative balance or equilibrium. The basic idea
is that when a state of imbalance occurs (e.g. through a sudden substantial rise in
body temperature) something must happen to correct the imbalance and restore
equilibrium (e.g. sweating). In this case, the animal does not have to 'do' anything
because sweating is a completely automatic biological response (physiological).
However, in the case of an imbalance caused by the body's need for food or drink
(tissue need), the hungry or thirsty animal has to behave in a manner that will
procure food or water. It is here that the concept of a homeostatic drive becomes
important: Tissue need leads to internal imbalance, which leads to homeostatic
drive, which leads to appropriate behaviour, which leads to restoration of internal
balance, which leads to a reduction of the drive.
The term 'homeostasis'
is now used to cover all the co-ordinated processes by which
each organism maintains itself as a functional part of an ecosystem in a steady state.
This statement implies that organisms are able to 'perceive' an end point to which they
adjust their activities. These activities are manifest at the organ, system, species and
social levels, but ultimately there is a basic cellular reaction (or reactions) which
underlies these adjustments. Thus, for all organisms, there are optimum conditions for
life which at any point in time may be expressed in terms of the composition of the
internal environment. We talk about regulatory mechanisms whereby organisms
minimize the internal effects of environmental changes in, for example, temperature and
salt content on the one hand, and population density on the other. The properties of the
internal environment always change less than those of the external environment.
However, the presence of homeostatic mechanisms does not imply a lack of change
because the end point or value for the optimum steady state condition may shift with
time.
The study of homeostatic
mechanisms leads to an understanding of how different
organisms are able to live and reproduce under adverse conditions. The term 'adverse'
is used in the sense that no organism can maintain itself in any environment without
effort. Problems of homeostasis are not so marked in a temperate climate as they are in
polar or equatorial regions, but the problems encountered in the latter areas are merely
temperate problems magnified many times, but in different environmental directions.
There are three theoretical
ways in which homeostasis may be accomplished.
(1) If the environmental
change is predictable, a timed device could provide a periodic
internal counterpoise to the known fluctuation.
(2) The external conditions
could be assessed and, together with a knowledge of the
properties of the reacting system, an estimate could be made as to the extent of the
anticipated change. An appropriate response could then be initiated to counteract the
expected change.
(3) The internal condition
could be monitored and any undue departure from a desirable
norm could be used as a signal to initiate a response which would stop only when the
norm had been restored.
