Cambridge International A Level Biology Revision Guide

Chapter 14: Homeostasis
Homeostatic control
Most control mechanisms in living organisms use a
negative feedback control loop (Figure 14.2) to maintain
homeostatic balance. This involves a receptor (or sensor)
and an effector. Effectors include muscles and glands.
The receptor detects stimuli that are involved with the
condition (or physiological factor) being regulated. A
stimulus is any change in a factor, such as a change in
blood temperature or the water content of the blood. The
body has receptors which detect external stimuli and other
receptors that detect internal stimuli. These receptors send
information about the changes they detect through the
nervous system to a central control in the brain or spinal
cord. This sensory information is known as the input. The
central control instructs an effector to carry out an action,
which is called the output. These actions are sometimes
called corrective actions as their effect is to correct (or
reverse) the change. Continuous monitoring of the factor
by receptors produces a steady stream of information to
the control centre that makes continuous adjustments
to the output. As a result, the factor fluctuates around a
particular ‘ideal’ value, or set point. This mechanism to
keep changes in the factor within narrow limits is known
as negative feedback. In these systems, an increase in
the factor results in something happening that makes
the factor decrease. Similarly, if there is a decrease in
the factor, then something happens to make it increase.
Homeostatic mechanisms involve negative feedback as it
minimises the difference between the actual value of the
factor and the ideal value or set point. The factor never
stays exactly constant, but fluctuates a little above and a
little below the set point.
The homeostatic mechanisms in mammals require
information to be transferred between different parts of
the body. There are two coordination systems in mammals
that do this: the nervous system and the endocrine system.
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In the nervous system, information in the form of
electrical impulses is transmitted along nerve cells
(neurones).
The endocrine system uses chemical messengers called
hormones that travel in the blood, in a form of longdistance
cell signalling.
QUESTION
14.1 a Describe the immediate environment of a typical
cell within the body of a mammal.
b Explain why it is important that the internal
environment of a mammal is carefully regulated.
c Explain how the following are involved in
maintaining the internal environment of a
mammal: stimuli, receptors, central control,
coordination systems and effectors.
d i Explain the meaning of the terms homeostasis
and negative feedback.
ii Distinguish between the input and the output
in a homeostatic control mechanism.
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Factor rises
above set point.
Receptors sense
change in factor.
Factor falls
below set point.
Effectors act to
increase factor.
Effectors act to
decrease factor.
Effectors receive information
from receptor.
Figure 14.2 A negative feedback control loop.