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.
