FROM PENN VALLEY COMMUNITY COLLEGE
HOMEOSTASIS {hohm-ee-oh-stay'-sis}
Homeostasis is the maintenance of equilibrium, or constant conditions, in a biological
system by means of automatic mechanisms that counteract influences tending toward
disequilibrium. The development of the concept, which is one of the most fund mental in
modern biology, began in the 19th century when the French physiologist Claude BERNARD
noted the constancy of chemical composition and physical properties of blood and other
body fluids. He claimed that this "fixity of the milieu interieur" was essential
to the life of higher organisms. The term homeostasis was coined by the 20th-century
American physiologist Walter B. Cannon, who refined and extended the concept of
self-regulating mechanisms in living systems.
Homeostatic mechanisms operate at all levels of organization in living systems, including
the molecular, cellular, organismic, and even populational levels. In complex organisms,
such as humans, it involves constant monitoring and regulating of numerous factors,
including the gases oxygen and carbon dioxide, nutrients, hormones, and organic and
inorganic substances. The concentrations of these substances in body fluid remain
unchanged, within limits, despite changes in the external environment.
At the molecular level, a homeostatic mechanism called feedback inhibition operates to
limit the amount of chemical product produced by an enzyme system. An enzyme system
consists of several enzymes that act sequentially to convert a metabolite into an end
product which the organism needs. Overproduction of the end product is prevented by the
inhibitory effect of the end product on the first enzyme in the sequence, the regulatory
enzyme. As the end product is used up in subsequent metabolic conversions, however, its
inhibitory effect on the regulatory enzyme decreases, so that more end product can be
formed by the enzyme system. In this manner, the level of end product is maintained at a
fairly constant level.
An example of homeostasis in cells is the phenomenon called contact inhibition, in which
division in a population of cells stops when they become so numerous that they touch each
other. It is believed that, a chemical "messenger" that inhibits further cell
division is passed from cell to cell. In contrast, cultured, or artificially produced,
cancer cells continue dividing even after cells touch. Thus, cancer cells appear to have
lost the homeostatic mechanism of contact inhibition. Homeostasis in organisms is
exemplified by the operations of the endocrine system. The hormone-synthesizing activities
of the endocrine glands are regulated by events occurring in the systems that the hormones
regulate. For example, a rise of blood-glucose levels stimulates the pancreas to secrete
insulin, which acts to accelerate the removal of glucose from the blood by conversion into
the storage products glycogen and fat. The sensations of hunger and thirst are also
homeostatic mechanisms; they help the organism maintain optimum levels of energy,
nutrients, and water.
Homeostatic mechanisms also operate to regulate the size of populations. An example is the
relationship between the populations of predatory animals and their prey. If prey become
abundant, so do their predators, until predation diminishes the supply of prey and causes
a decline in the predator population. This allows the prey population to build up again,
and the cycle is repeated. In this manner, the populations of both kinds of animals
oscillate around a mean.
Peter L. Petrakis
Bibliography:
Pribor, Donald B., Functional Homeostasis (1986)
Trojan, P., Ecosystem Homeostasis (1984).
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Based on Information from School of Biochemistry at
LaTrobe University:
What is homeostasis?
How is homeostasis achieved?
Feedback mechanisms
Feedback loops
WHAT IS HOMEOSTASIS?
Homeostasis is the maintenance of a constant
internal environment (the immediate surroundings of cells) in response to changes in:
the changing conditions of the external
environment.
the changing conditions of the internal environment.
Homeostasis is a self adjusting mechanism involving
feedback where the response to a stimulus alters the internal conditions and may itself
become a new stimulus.
Homeostasis works to maintain the organism's
internal environment within tolerance limits - the narrow range of conditions where
cellular processes are able to function at a level consistent with the continuation of
life.
HOW IS HOMEOSTASIS ACHIEVED?
To maintain cells, tissues and entire organisms
within their biological tolerance limits, various mechanisms have evolved. These may be
structural: the animal or plant has particular
physical features which help its survival in an otherwise hostile environment.
functional: the metabolism of the animal or plant
is able to adjust to changes in conditions as they are detected.
behavioral: the actions and interactions of the
individual, either alone or with others, help it to survive in its particular environment.
Homeostasis is really the combined result of all of
these, a failure of any one of them can result in the death of an individual.
FEEDBACK MECHANISMS
Feedback mechanisms are the general mechanism of
nervous or hormonal regulation in animals. Essentially, feedback occurs when the response
to a stimulus has an effect of some kind on the original stimulus. The nature of the
response determines how the feedback is 'labeled'.
Negative feedback is when the response diminishes
the original stimulus. Positive feedback is when the response enhances the original
stimulus.
Negative feedback is most common in biological
systems. Examples of this are:
Blood glucose concentrations rise after a sugary
meal (the stimulus), the hormone insulin is released and it speeds up the transport of
glucose out of the blood and into selected tissues (the response), so blood glucose
concentrations decrease (thus decreasing the original stimulus).
Exercise creates metabolic heat which raises the
body temperature (the stimulus), cooling mechanisms such as vasodilation (flushed skin)
and sweating begin (the response), body temperature falls (thus decreasing the original
stimulus).
Positive feedback is less common, which is
understandable, as most changes to steady state pose a threat, and to enhance them would
be most unhelpful. However, there are a few examples:
A baby begins to suckle her mother's nipple and a
few drops of milk are released (the stimulus). This encourages the baby and releases a
hormone in the mother which further stimulates the release of milk (the response). The
hungry baby continues to suckle, stimulating more milk release until she stops. (Positive
feedback, it would not have helped the baby if suckling decreased milk flow, as in
negative feedback!)
A ripening apple releases the volatile plant
hormone ethylene (the stimulus). Ethylene accelerates the ripening of unripe fruit in its
vicinity so nearby fruit also ripens, releasing more ethylene (the response). All the
fruit quickly becomes ripe together. ("One 'bad' apple has ruined the whole
lot." The biological explanation - positive feedback - for an old saying!)
FEEDBACK LOOPS
Regardless of whether the feedback is positive or
negative, feedback mechanisms have certain essential components.
Students should be able to identify each of these
and explain their role.
Stimulus: The change from ideal or resting
conditions.
Receptor: The cells or tissue which detects the
change due to the stimulus.
Relay: The transmission of the message, via nerves
or hormones or both, to the effector.
Effector: The cells or tissue, usually a gland or
muscles, which cause the response to happen.
Response: An action, at cell, tissue or whole
organism level which would not have occurred in the absence of the stimulus.
Feedback: The consequence of the response on the
stimulus. May be positive or negative.
These feedback loops, as they are often called, are
usually well illustrated in textbooks.
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