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.
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). Thsi 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!)
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