WHAT IS HOMEOSTASIS?
Homeostasis is the maintenance of a constant internal environment (the immediate
surroundings of cells) in response to changes in:
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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
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structural: the animal or plant has particular physical features which help its survival
in an otherwise hostile environment.
functional: the metaboilsm of the animal or plant is able to adjust to changes in
conditions as they are detected. (The focus of this Area of Study).
behavioural: the actions and interactions of the individual, either alone or with
others, help it to survive in its particular environment. (Part of Area of Study 3.)
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. For now, we will focus on functional mechanisms of
homeostasis, but students should not consider that these in isolation will maintain 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
'labelled'.
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:
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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:
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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 vacinity 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.
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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.
Check yours for examples, or click here to see some feedback loop diagrams