10
UNIT 1
Organization of the Body
1
Positive Feedback Mechanisms
In
positive feedback mechanisms
, the result or response en-
hances the original stimulus so that the response is accelerated.
Tis feedback mechanism is “positive” because the change that
results proceeds in the
same
direction as the initial change, caus-
ing the variable to deviate further and further from its original
value or range.
In contrast to negative feedback controls, which maintain
some physiological function or keep blood chemicals within
narrow ranges, positive feedback mechanisms usually con-
trol infrequent events that do not require continuous adjust-
ments. ±ypically, they set off a series of events that may be
self-perpetuating and that, once initiated, have an amplifying
or waterfall effect. Because of these characteristics, positive
feedback mechanisms are oFen referred to as
cascades
(from
the Italian word meaning “to fall”). ±wo familiar examples of
their use as homeostatic mechanisms are the enhancement of la-
bor contractions during birth and blood clotting.
Te endocrine system is equally important in maintaining
homeostasis. A good example of a hormonal negative feedback
mechanism is the control of blood sugar (glucose) by insulin. As
blood sugar rises, receptors in the body sense this change, and the
pancreas (the control center) secretes insulin into the blood. Tis
change in turn prompts body cells to absorb more glucose, re-
moving it from the bloodstream. As blood sugar falls, the stimulus
for insulin release ends. Te body’s ability to regulate its internal
environment is fundamental. All negative feedback mechanisms
have the same goal: preventing sudden severe changes within the
body. Body temperature and blood volume are only two of the
variables that need to be regulated. Tere are hundreds! Other
negative feedback mechanisms regulate heart rate, blood pres-
sure, the rate and depth of breathing, and blood levels of oxygen,
carbon dioxide, and minerals. Now, let’s take a look at the other
type of feedback control mechanism—positive feedback.
Sweat glands activated
Shivering begins
Body temperature
rises
Stimulus: Heat
BALANCE
IMBALANCE
IMBALANCE
Afferent
pathway
Afferent
pathway
Efferent
pathway
Efferent
pathway
Stimulus: Cold
Body temperature
falls
Receptors
Temperature-sensitive
cells in skin and brain
Receptors
Temperature-sensitive
cells in skin and brain
Effectors
Sweat glands
Effectors
Skeletal muscles
Control Center
(thermoregulatory
center in brain)
Control Center
(thermoregulatory
center in brain)
Response
Evaporation of sweat
Body temperature falls;
stimulus ends
Response
Body temperature rises;
stimulus ends
Figure 1.5
Body temperature is regulated by a negative feedback mechanism.
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