Respiratory Physiology
479
Chronic medical problems
and daytime sleepiness
Arterial
P
O
2
Arterial
P
CO
2
Frequent arousal
and sleep disruption
Closure of airway during sleep
Compromised upper airway
anatomy and increased
compliance
Decreased upper airway
dilator muscle activity
Obesity
Figure 13–44
Pathogenesis of obstructive sleep apnea.
During normal sleep, air flows freely
past the structures in the throat.
During sleep apnea, air flow is
completely blocked.
With CPAP, a mask over the nose
gently directs air into the throat to
keep the air passage open.
Air flow
Soft palate
Base of
the tongue
Uvula
Tonsils
(a)
(b)
(c)
Figure 13–45
The pathophysiology and a standard treatment of obstructive sleep apnea. (a) Normal sleep with air fl owing freely past the structures of the
throat during an inspiration. (b) In obstructive sleep apnea (particularly with the patient sleeping in the supine position), the soft palate, uvula,
and tongue occlude the airway, greatly increasing the resistance to air fl ow. (c) Continuous positive airway pressure (CPAP) is applied with a
nasal mask preventing airway collapse.
SUMMARY
Organization of the Respiratory System
I. The respiratory system comprises the lungs, the airways leading
to them, and the chest structures responsible for moving air
into and out of them.
a. The conducting zone of the airways consists of the trachea,
bronchi, and terminal bronchioles.
b. The respiratory zone of the airways consists of the alveoli,
which are the sites of gas exchange, and those airways to
which alveoli are attached.
c. The alveoli are lined by type I cells and some type II cells,
which produce surfactant.
d. The lungs and interior of the thorax are covered by pleura;
between the two pleural layers is an extremely thin layer of
intrapleural fl
uid.
II. The lungs are elastic structures whose volume depends upon
the pressure difference across the lungs—the transpulmonary
pressure—and how stretchable the lungs are.
III. The steps involved in respiration are summarized in Figure 13–6.
In the steady state, the net volumes of oxygen and carbon
dioxide exchanged in the lungs per unit time are equal to the
net volumes exchanged in the tissues.
Ventilation and Lung Mechanics
I. Bulk fl ow of air between the atmosphere and alveoli is
proportional to the difference between the alveolar and
atmospheric pressures and inversely proportional to the airway
resistance:
F
= (
P
alv
P
atm
)/
R
.
II. Between breaths at the end of an unforced expiration
P
atm
=
P
alv
,
no air is fl owing, and the dimensions of the lungs and thoracic
cage are stable as the result of opposing elastic forces. The lungs
are stretched and are attempting to recoil, whereas the chest wall
is compressed and attempting to move outward. This creates a
subatmospheric intrapleural pressure and hence a transpulmonary
pressure that opposes the forces of elastic recoil.
III. During inspiration, the contractions of the diaphragm and
inspiratory intercostal muscles increase the volume of the
thoracic cage.
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