THE PLANNER
T
a.
Feedback
mechanism
b. Negative feedback
(Example: blood pressure regulation)
c. Positive feedback
(Example: labor)
Some stimulus
disrupts homeostasis by
Increasing
Blood pressure
Return to homeostasis
when response brings
blood pressure
back to normal
Response that alters the
controlled condition
The effector’s response restores the controlled
condition
to n o rm a l. Negative feedback:
Decreased
heart rate decreases blood pressure back to normal.
Positive feedback:
Increased uterine contractions cause
more stretching and subsequent release of oxytocin.
Contractions of wall
of uterus force baby's
head or body into
the cervix, thus
Increasing
Stretching of cervix
Receptors
Stretch-sensitive nerve
cells in cervix send
Input
Nerve
impulses to a
Control center
Brain interprets.,,
input 'and releases
Positive feedback:
Increased stretching of
cervix causes release
of more oxytocin,
which results in more
stretching of the cervix
Baby's body stretches
cervix more
Interruption of cycle:
Birth of baby decreases
stretching of cervix,
thus breaking the
positive feedback cycle
Put It T°gethe|f
To which of the following does
a negative feedback system lead?
a. a stable controlled condition
b. an unstable controlled condition
feedback systems tend to maintain stable conditions, whereas
positive feedback systems tend to be unstable and must be
shut off by some event that is outside the feedback loop, such
as the delivery of the child.
Homeostasis is
dynamic
; that is, it can change over a nar-
row range that is compatible with maintaining cellular life
processes. Your physiological state
can move from one
steady state
to
another,
depending
upon various
stimuli. For example, when you are
resting, your body is in one steady
state of conditions, but when you are exercising, your
homeostatic mechanisms adapt to create another steady
state. As a result of the many feedback mechanisms, most
disruptions of homeostasis are mild and temporary; the
responses of body cells quickly restore balance in the in-
ternal environment. For example, when you stand, your
blood pressure momentarily drops, but it is restored to
normal within a heartbeat or two. (If this did not happen,
you would faint.) In other cases, the disruptions of homeo-
stasis may be intense and prolonged, such as with severe
infection or poisoning.
steady state
A set
of conditions that
remains constant over
some period of time.
All Living Organisms Carry Out Common Life Processes
9
PROCESS DIAGRAM
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