Chapter 1
The Human Body: An Orientation
9
1
pathway.
(To help you remember the difference between “affer-
ent” and “efferent,” you might note that information traveling
along the afferent pathway
approaches
the control center and ef-
ferent information
exits
from the control center.)
±e
effector
provides the means for the control center’s re-
sponse (output) to the stimulus. ±e results of the response then
feed back
to influence the effect of the stimulus, either reducing
it (in negative feedback) so that the whole control process is
shut off, or enhancing it (in positive feedback) so that the whole
process continues at an even faster rate.
Negative Feedback Mechanisms
Most homeostatic control mechanisms are
negative feedback
mechanisms
. In these systems, the output shuts off the original
effect of the stimulus or reduces its intensity. ±ese mechanisms
cause the variable to change in a direction
opposite
to that of the
initial change, returning it to its “ideal” value; thus the name
“negative” feedback mechanisms.
Let’s start with an example of a nonbiological negative feed-
back system: a home heating system connected to a temperature-
sensing thermostat. ±e thermostat houses both the receptor
(thermometer) and the control center. If the thermostat is set at
20°C (68°F), the heating system (effector) is triggered ON when
the house temperature drops below that setting. As the furnace
produces heat and warms the air, the temperature rises, and
when it reaches 20°C or slightly higher, the thermostat triggers
the furnace OFF. ±is process results in a cycling of “furnace-
ON” and “furnace-OFF” so that the temperature in the house
stays very near the desired temperature of 20°C. Your body
“thermostat,” located in a part of your brain called the hypotha-
lamus, operates in a similar fashion
(Figure 1.5)
.
Regulation of body temperature is only one of the many ways
the nervous system maintains the constancy of the internal envi-
ronment. Another type of neural control mechanism is seen in the
withdrawal reflex
mentioned earlier, in which the hand is jerked
away from a painful stimulus such as broken glass.
narrow limits. In general, the body is in homeostasis when its
needs are adequately met and it is functioning smoothly.
Maintaining homeostasis is more complicated than it ap-
pears at first glance. Virtually every organ system plays a role
in maintaining the constancy of the internal environment.
Adequate blood levels of vital nutrients must be continuously
present, and heart activity and blood pressure must be con-
stantly monitored and adjusted so that the blood is propelled to
all body tissues. Also, wastes must not be allowed to accumulate,
and body temperature must be precisely controlled. A wide va-
riety of chemical, thermal, and neural factors act and interact in
complex ways—sometimes helping and sometimes hindering
the body as it works to maintain its “steady rudder.”
Homeostatic Control
Communication within the body is essential for homeostasis.
Communication is accomplished chiefly by the nervous and en-
docrine systems, which use neural electrical impulses or blood-
borne hormones, respectively, as information carriers. We cover
the details of how these two great regulating systems operate in
later chapters, but here we explain the basic characteristics of
control systems that promote homeostasis.
Regardless of the factor or event being regulated—the
variable
—all homeostatic control mechanisms are processes
involving at least three components that work together
(Figure 1.4)
. ±e first component, the
receptor
, is some type of
sensor that monitors the environment and responds to changes,
called
stimuli
, by sending information (input) to the second
component, the
control center
. Input flows from the receptor to
the control center along the so-called
afferent pathway
.
±e
control center
determines the
set point
, which is the level
or range at which a variable is to be maintained. It also analyzes
the input it receives and determines the appropriate response or
course of action. Information (output) then flows from the con-
trol center to the third component, the
effector
, along the
efferent
Stimulus
produces
change in
variable.
Receptor
detects change.
Input:
Information
sent along afferent
pathway to control
center.
Output:
Information
sent along efferent
pathway to effector.
Response
of effector feeds
back to reduce
the effect of
stimulus and
returns variable
to homeostatic
level.
Receptor
Effector
Control
Center
1
2
3
4
5
BALANCE
IMBALANCE
IMBALANCE
Afferent
pathway
Efferent
pathway
Figure 1.4
Interactions among the elements of a homeostatic control system maintain
stable internal conditions.
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