Homeostatic Balance Maintains Relatively
Stable Conditions Inside the Body
You would not feel well if your body temperature changed from
normal one moment to high fever in the next followed by chills
in the moment after that. Likewise, you would be in serious dan-
ger if your heart rate changed errati-
cally and spontaneously every minute.
Your body and its cells require relatively
stable conditions; the maintenance of
these stable conditions is called
homeo-
stasis
. Homeostasis protects the inter-
nal environment against changes both
inside and outside the body.
Every body structure, from cells to systems, has one or more
homeostatic mechanisms, which are mainly under the control of
two systems:
The nervous system detects changes from the balanced state
and sends messages in the form of nerve impulses to organs that
can counteract them. For example, when body temperature
rises, nerve impulses cause sweat glands to secrete more
sweat, which evaporates and cools the body.
The
endocrine
system
corrects
changes
by
secreting
chemicals
called
hormones
into
the
blood.
Hormones affect specific body cells
where
they cause
responses
that
restore homeostasis. For example,
the
hormone
insulin reduces the
blood glucose level when it is too
high.
Nerve impulses typically cause rapid cor-
rections, while hormones usually work
more slowly.
Homeostasis
is
maintained
by
many
feedback systems
(
Figure 1.4
).
Each monitored condition in a feedback system, or
feedback loop,
is termed a
co ntro lled co n d itio n
. Any disruption that causes a
change in a controlled condition is called a
stimulus.
Some stim-
uli come from outside the body, while others come from within.
In addition to the controlled condition and the stimulus,
feedback systems have three other components:
1.
A
receptor
monitors the controlled condition and sends
information (input) to a control center.
2.
A
control center
receives the input, compares it to a set of
values that the controlled condition should have (set point)
and sends output commands (nerve impulses or chemical
signals) to an effector.
3.
An
effector
receives output commands and produces a
response that changes the controlled condition.
hormone
(HOR-mon)
A secretion of endo-
crine cells that alters
the physiological
activity of target cells
of the body.
feedback system
A sequence of events
in which information
about the status of a
situation is continually
reported (fed back) to
a control center.
homeostasis
(ho-me-o-STA-sis) The
condition in which the
body's internal environ-
ment remains relatively
constant, within physi-
ological limits.
Feedback systems • Figure 1.4
Characteristics of a feedback system a., how a negative
feedback system controls blood pressure b., and how a
positive feedback system operates in labor c.
Some stimulus alters homeostasis by altering
a
controlled condition.
Negative feedback:
the
stimulus increases blood pressure.
Positive feedback:
Contractions of the uterus stretch the cervix.
3
A
receptor senses
a
change in the controlled
condition and provides input
to a
control center.
Negative feedback:
Pressure receptors in certain blood
vessels send nerve impulses to the brain.
Positive feedback:
Stretch receptors in the cervix send signals to the brain.
The control center receives the input, compares
it
to
some set point, and sends output
to an
effector.
Negative feedback:
The brain compares the
increase in blood pressure to the normal value and
sends nerve impulses to the heart.
Positive feedback:
The brain (hypothalamus) interprets the impulses and
releases oxytocin.
a
Th e effector receives the output and produces a
response.
Negative feedback:
The heart receives nerve
impulses to decrease its rate of beating.
Positive feedback:
Uterine m uscles contract.
If a feedback system reverses the change in the controlled
condition to restore it to the set point, this is a
negative
feedback system
; for example, a negative feedback
system controls blood pressure. However, if a feedback
system further strengthens a change in the controlled
condition, this is a
positive feedback system
. For ex-
ample, childbirth is an example of positive feedback.
During labor, uterine contractions force the baby’s head
into the cervix, which stretches. The stretching causes
the hypothalamus to secrete a hormone called oxyto-
cin, which induces more uterine contractions. Negative
8 CHAPTER 1
Organization of the Human Body
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