BIOL 2011L Lecture Notes - Lecture 4: Blood Sugar, Extracellular Fluid, Fluid Balance

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26 Nov 2017
Homeostasis can be defined as the stable condition of an organism and of its internal
environment; or as the maintenance or regulation of the stable condition, or its equilibrium;
or simply as the balance of bodily functions.The stable condition is the condition of optimal
functioning for the organism, and is dependent on many variables, such as body temperature
and fluid balance, being kept within certain pre-set limits. Other variables include the pH of
extracellular fluid, the concentrations of sodium, potassium and calcium ions, as well as that
of the blood sugar level, and these need to be regulated despite changes in the environment,
diet, or level of activity. Each of these variables is controlled by one or more regulators or
homeostatic mechanisms, which together maintain life.
Homeostasis is brought about by a natural resistance to change in the optimal
conditions,and equilibrium is maintained by many regulatory mechanisms. All homeostatic
control mechanisms have at least three interdependent components for the variable being
regulated: a receptor, a control centre, and an effector. The receptor is the sensing
component that monitors and responds to changes in the environment, either external or
internal. Receptors include thermoreceptors, and mechanoreceptors. Control centres
include the respiratory centre, and the renin-angiotensin system. An effector is the target
acted on, to bring about the change back to the normal state. At the cellular level, receptors
include nuclear receptors that bring about changes in gene expression through up-
regulation or down-regulation, and act in negative feedback mechanisms. An example of this
is in the control of bile acids in the liver.
Some centres such as the renin-angiotensin system, control more than one variable. When
the receptor senses a stimulus, it reacts by sending an action potential to a control centre.
The control centre sets the maintenance range, the acceptable upper and lower limits, for
the particular variable such as temperature . The control center responds to the signal by
determining an appropriate response and sending signals to an effector which can be one or
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