757
CHAPTER NINETEEN
Respiratory System
PRACTICE
31
Where are the respiratory areas?
32
Describe how the respiratory areas maintain a normal breathing
pattern.
33
Explain how the breathing pattern may be changed.
Factors Affecting Breathing
In a mixture of gases such as air, each gas accounts for a por-
tion of the total pressure the mixture produces. The amount
of pressure each gas contributes is called the
partial pres-
sure
of that gas and is proportional to its concentration. For
example, because air is 21% oxygen, oxygen accounts
for 21% of the atmospheric pressure (21% of 760 Hg), or
160 mm Hg (.21 × 760 = 160). Thus, the partial pressure
of oxygen, symbolized P
O
2
, in atmospheric air is 160 mm Hg.
Similarly, the partial pressure of carbon dioxide (P
CO
2
) in air
is 0.3 mm Hg.
Gas molecules from the air may enter, or dissolve, in liq-
uid. This is what happens when CO
2
is added to a carbonated
beverage or when inspired gases dissolve in the blood in the
alveolar capillaries. Using partial pressures simpli± es the cal-
culation of the concentration of a dissolved gas. The partial
pressure of a gas dissolved in a liquid equals the partial pres-
sure of that gas in the air the liquid has equilibrated with. For
example, the P
O
2
in a glass of water that has been on your desk
for awhile must be 160mm Hg, the same as in the air around
it. Thus, instead of concentrations of oxygen and carbon diox-
ide in the body fl uids, we will refer to P
O
2
and P
CO
2
.
A number of factors infl
uence breathing rate and depth.
These include P
O
2
and P
CO
2
in body fl
uids, the degree to which
lung tissues are stretched, emotional state, and level of
physical activity. The receptors involved include mechano-
receptors that sense stretch as well as central and peripheral
chemoreceptors.
Central chemoreceptors
are in chemosensitive areas in
the ventral portion of the medulla oblongata near the ori-
gin of the vagus nerve. These chemoreceptors respond to
changes in blood pH, but only indirectly, because hydrogen
ions do not easily cross the blood-brain barrier. However,
if plasma P
CO
2
rises, the CO
2
easily diffuses into the brain,
where it combines with water in the cerebrospinal fl uid to
form carbonic acid (H
2
CO
3
):
CO
2
+ H
2
O
H
2
CO
3
The carbonic acid soon ionizes, releasing hydrogen ions (H
+
)
and bicarbonate ions (HCO
3
):
H
2
CO
3
H
+
+ HCO
3
Hydrogen ions rather than carbon dioxide influence the
central chemoreceptors. Breathing rate and tidal volume
increase when a person inhales air rich in CO
2
or when body
cells produce excess CO
2
. These changes increase alveolar
ventilation. More CO
2
is exhaled, and the blood P
CO
2
and
hydrogen ion concentration return toward normal.
on the experimental conditions. Indeed, it has been sug-
gested that such an important physiological process might
have more than one way of occurring.
The
dorsal respiratory group
stimulates the inspiratory
muscles, primarily the diaphragm, and may contribute to
more forceful breathing. The dorsal respiratory goup also
helps process sensory information regarding the respiratory
system.
Neurons in another part of the brainstem, the pons, com-
pose the
pontine respiratory group
(formerly the
pneumo-
taxic center
). They may contribute to the rhythm of breathing
by limiting inspiration
(f
g. 19.29)
.
A condition called
sleep apnea
is responsible for some cases of sud-
den infant death and for snoring. Infant apneas are almost always
central,
due to a problem with respiratory control centers. In adults,
these apneas are usually
obstructive,
involving airway blockage,
Babies who have diF
culty breathing just after birth are often sent
home with monitoring devices, which sound an alarm when the child
stops breathing, alerting parents to resuscitate the infant. The posi-
tion in which the baby sleeps seems to a±
ect the risk of sleep apnea—
sleeping on the back or side is safest during the ²
rst year of life.
Adults with sleep apnea may cease breathing for ten to twenty
seconds, hundreds of times a night. Bedmates may be aware of the
problem because the frequent cessation in breathing causes snoring.
The greatest danger of adult sleep apnea is the fatigue, headache,
depression, and drowsiness that follows during waking hours.
Sleep apnea is diagnosed in a sleep lab, which monitors breath-
ing during slumber. One treatment for obstructive sleep apnea is
nasal continuous positive airway pressure.
A device is strapped onto
the nose at night that maintains air ³
ow into the respiratory system.
Medullary respiratory center
Respiratory muscles
Dorsal
respiratory
group
Forceful breathing
Basic rhythm
of breathing
Nerve impulses
Nerve impulses
Ventral
respiratory
group
Respiratory areas
Pontine respiratory
group
FIGURE 19.29
The medullary respiratory center and the pontine
respiratory group control breathing.
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