758
UNIT FIVE
gen it had when it was fully oxygenated. This large excess of
oxygen “frees up” respiratory control from paying attention
to blood oxygen levels under most circumstances. Thus, the
respiratory system can focus on blood P
CO
2
and hydrogen ion
concentration, which are important in maintaining the pH of
the internal environment.
The peripheral chemoreceptors of the carotid and aortic
bodies are also stimulated by changes in the blood P
CO
2
and
pH. However, CO
2
and hydrogen ions have a much greater
effect on the central chemoreceptors of the respiratory center
than they do on the carotid and aortic bodies, although this
relationship may change with intense exercise.
Patients who have chronic obstructive pulmonary diseases (COPD),
such as asthma, bronchitis, and emphysema, gradually adapt to high
concentrations of carbon dioxide. For them, low oxygen concentra-
tions may serve as a necessary respiratory stimulus. When such a
patient is placed on 100% oxygen, the low arterial P
O
2
may be cor-
rected, the stimulus removed, and breathing may stop.
An
infl
ation refl
ex
(Hering-Breuer refl ex) helps regulate
the depth of breathing. This refl
ex occurs when stretch recep-
tors in the visceral pleura, bronchioles, and alveoli are stim-
ulated as lung tissues are stretched. The sensory impulses
of the refl ex travel via the vagus nerves to the pontine respi-
ratory group and shorten the duration of inspiratory move-
ments. This action prevents overinfl
ation of the lungs during
forceful breathing
(f
g. 19.31)
.
Emotional upset or strong sensory stimulation may alter
the normal breathing pattern. Gasping and rapid breathing
are familiar responses to fear, anger, shock, excitement, hor-
ror, surprise, sexual stimulation, or even the chill of stepping
into a cold shower. Control of the respiratory muscles is vol-
untary, so we can alter breathing pattern consciously or stop
it altogether for a short time. During childbirth, for example,
women often concentrate on controlling their breathing,
which distracts them from the pain.
If a person decides to stop breathing, the blood con-
centrations of carbon dioxide and hydrogen ions begin to
rise, and the concentration of oxygen falls. These changes
(primarily the increased CO
2
) stimulate the chemorecep-
tors, and soon the need to inhale overpowers the desire to
hold the breath—much to the relief of parents when young
children threaten to hold their breath until they turn blue!
However, a person can increase the breath-holding time
by breathing rapidly and deeply in advance. (This could
be dangerous, see box that follows.) This action, termed
hyperventilation
(hi
per-ven
tı˘-la
shun), lowers the blood
CO
2
concentration below normal. Following hyperven-
tilation, it takes longer than usual for the carbon dioxide
concentration to reach the level needed to override the con-
scious effort of breath holding.
Table 19.6
discusses factors affecting breathing. Clinical
Application 19.4 focuses on one infl uence on breathing—
exercise.
Adding CO
2
to air can stimulate the rate and depth of breathing.
Ordinary air is about 0.04% CO
2
. If a patient inhales air containing 4%
CO
2
, breathing rate usually doubles.
Low blood P
O
2
has little direct effect on the central
chemoreceptors associated with the medulla oblongata.
Instead, changes in the blood P
O
2
are primarily sensed by
peripheral chemoreceptors
in specialized structures called
the
carotid bodies
and
aortic bodies,
located in the walls
of the carotid sinuses and aortic arch
(±ig. 19.30)
. When
decreased P
O
2
stimulates these peripheral receptors, impulses
are transmitted to the respiratory center, and the breathing
rate and tidal volume increase, increasing alveolar ventila-
tion. This mechanism does not usually play a major role until
the P
O
2
decreases to about 50% of normal. Therefore, oxygen
plays only a minor role in the control of normal respiration.
The limited role of P
O
2
may be surprising, considering the
importance of oxygen for sustaining life. Because most blood
oxygen is carried on the hemoglobin in red blood cells, deox-
ygenated systemic venous blood still has 75% of the oxy-
Carotid bodies
Common carotid
artery
Sensory nerve
(branch of vagus nerve)
Aorta
Heart
Aortic bodies
Sensory nerve
(branch of
glossopharyngeal
nerve)
Medulla oblongata
FIGURE 19.30
±Decreased±P
O
2
stimulates peripheral chemoreceptors
in the carotid and aortic bodies.
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