The Breathing Control Centers
Are Influenced by Many Factors
Certain chemical stimuli determine how quickly and how
deeply we breathe. The respiratory system functions to
maintain proper levels of CO2
and O2
and is very respon-
sive to changes in the level of either in body fluids. Sen-
sory neurons that are responsive to chemicals are called
chemoreceptors.
Chemoreceptors in the medulla, in the carotid bod-
ies of the carotid artery, and in the aortic arch influence
the respiratory centers through reflex pathways (Figure
13.8). The chemoreceptors in the medulla, called central
chemoreceptors, monitor the PCO
and pH of the cere-
brospinal fluid (CSF). The chemoreceptors in the carotid
bodies and aortic arch, known as peripheral chemorecep-
tors, monitor the PO
, PCO
, and pH of the blood. Decreases
in PO
and pH or increases in PCO
cause the chemorecep-
tors to send nerve impulses to the respiratory center to
increase the rate of breathing. Conversely, increases in PO
and pH or decreases in PCO
reduce the impulses from the
chemoreceptors and reduce the breathing rate.
The chemoreceptors participate in a negative feedback
system that regulates the levels of CO2, O2, and H+ in the
blood. If these levels change, as shown in Figure 13.8, in-
put from the central and peripheral chemoreceptors cause
the inspiratory area to become highly active. The rate and
depth of breathing increase. Rapid and deep breathing,
called hyperventilation, allows the exhalation of more CO2
until PCO
and H+ are lowered to normal levels.
Severe deficiency of O2, however, depresses the activity
of the central chemoreceptors and inspiratory area. They
do not respond well to any inputs and send fewer inpulses
to the muscles of respiration. As the rate of breathing de-
creases or stops entirely, PO
falls lower and lower, with
possibly fatal results.
Remember the mountain-climbing example at the be-
ginning of this chapter? Initially, the body responds to high
altitudes with hyperventilation, as just described. How-
ever, full acclimatization may take days or weeks and in-
volves other body systems, including the kidneys. Even so,
humans cannot live for a long time without supplementary
CONCEPT CHECK
1.
Which
respiratory areas are involved in increas-
ing the rate of breathing?
2.
How
is a change in pH of the cerebrospinal fluid
detected?
oxygen sources at altitudes above 5,950 meters (19,520
feet), and cannot live at all at altitudes above 8.000 meters
(26,000 feet). This range, above 8,000 meters, is called the
"death zone" in mountaineering.
In addition to the chemical influences just described,
numerous other factors regularly impact the rate and
depth of breathing:
L im b ic system .
In response to emotional stresses, such
as anxiety and fear, the limbic system stimulates the
respiratory center to increase the rate and depth of
breathing.
Proprioreceptor stim u la tio n .
As soon as you start exercising,
your rate and depth of breathing increase—even before
changes in PO
PCO
, or H+ level occur. When joints
and muscles move (as in exercise), proprioreceptors
stimulate the inspiratory area to increase the rate and
depth of breathing.
Tem perature.
An increase in body temperature, which
can be caused by a fever or by exercise, increases
the rate of breathing. In contrast, a decrease in body
temperature decreases the rate of breathing. A sudden
cold stimulus (such as plunging into cold water) can
slow or even temporarily stop breathing (apnea).
P a in .
Sudden pain may induce apnea, but prolonged
somatic pain may increase breathing rate. Visceral
pain may slow breathing.
A ir w a y
irrita tio n .
Physical or chemical irritation of
the airways immediately stops breathing, followed by
coughing or sneezing to expel the irritants.
In fla tio n reflex.
Stretch receptors in the bronchi and
bronchioles inhibit the inspiratory area and induce
exhalation to prevent overinflation of the lungs. This
reflex is mainly a protective mechanism for preventing
excessive inflation of the lungs.
Defects or random impulses within the respiratory cen-
ter can also change some patterns of breathing. For ex-
ample, extra impulses from the respiratory center can
asynchronously stimulate the respiratory muscles and
cause hiccups.
3.
How
does an increase in PCO2 of the blood
stimulate breathing rate?
388 CHAPTER 13
The Respiratory System
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