CONCEPT CHECK
1.
What
drives oxygen from the alveoli into the
pulmonary capillaries?
2.
How
are oxygen and carbon dioxide exchanged
in the systemic capillaries?
3.
How
are oxygen and carbon dioxide transport-
ed in the blood?
The Brain Controls Breathing
LEARNING OBJECTIVES
1.
Associate
the various respiratory areas of the
brain with their roles in controlling respiration.
2.
Explain
the roles of chemoreceptors in influenc-
ing breathing.
t rest, about 200 mL of O2
are used each minute
by body cells. During strenuous exercise, how-
ever, O2
use typically increases 15- to 20-fold in
normal healthy adults, and as much as 30-fold
in elite endurance-trained athletes. Several mechanisms
help match respiratory efforts to metabolic demand.
As you learned in Chapter 11, your heart rate is set
internally in the heart muscle itself. In contrast, your
breathing rate is set externally by the respiratory center,
which consists of groups of neurons in the medulla oblon-
gata and pons in the brain.
Structures of the Brainstem
Regulate Breathing
The
r e sp ir a to r y c e n te r
consists of four spe-
cific areas that regulate breathing:
1.
The inspiratory area.
2.
The expiratory area.
3.
The
p n eu m o ta x ic area
(sometimes called
the
p o n tin e center)
of the pons.
4.
The
a p n e u stic a rea
.
The basic rate and depth of breathing is
set by the
in sp ir a to r y a re a
in the medulla
oblongata, which sends nerve impulses to the
muscles of inhalation (diaphragm, external in-
tercostals) to contract for two seconds. Impuls-
es then cease for three seconds, during which
3.
Outline
the negative feedback control of
breathing by central and peripheral chemore-
ceptors.
time the muscles relax and you exhale. This process oc-
curs continuously to set the rhythm of normal breathing.
The neurons of the
exp ira to ry a rea
in the medulla
oblongata remain inactive during quiet breathing. How-
ever, the inspiratory area can adjust the depth and rate of
breathing, based on input received from various body recep-
tors about gas and activity levels. When you need to breathe
harder, as during exercise, the inspiratory area sends nerve
impulses to additional muscles of inhalation and also sends
impulses to the expiratory muscles (internal intercostals,
abdominal muscles) for forceful exhalation.
Figure 1 3 .7
shows the relationships of the inspiratory and ex-
piratory areas during normal quiet breathing and
forceful breathing.
The pneumotaxic and apneustic areas in-
fluence the inspiratory area to modify the basic
rhythm of breathing. The pneumotaxic area can
shorten inspiration to create shorter, shallower
breaths (as in panting) or prolong inhalation to
allow slow, deep breaths (as with vocalization).
The cerebral cortex can also influence the
respiratory centers. Cortical impulses allow
you to voluntarily change the rate and depth of
breathing. You can even voluntarily hold your
breath, to prevent water or irritating gases
from entering the lungs. However, you can-
not hold your breath for very long. Even if you
hold your breath long enough to cause fainting,
breathing resumes when consciousness is lost.
The brainstem structures immediately take
p n eu m o ta x ic area
(noo-mo-TAK-sik) A
part of the respiratory
center in the pons that
cyclically sends inhibi-
tory nerve impulses to
the inspiratory area,
limiting inhalation and
facilitating exhalation.
a p n e u stic area
(ap-NOO-stik) A part
of the respiratory
center in the pons
that sends stimulatory
nerve impulses to the
inspiratory area that
activate and prolong
inhalation and inhibit
exhalation.
386 CHAPTER 13
The Respiratory System
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