592
SECTION VII
Respiratory Physiology
chest wall to recoil in the opposite direction. If the chest wall
is opened, the lungs collapse; and if the lungs lose their elastic-
ity, the chest expands and becomes barrel-shaped.
Inspiration is an active process. The contraction of the
inspiratory muscles increases intrathoracic volume. The
intrapleural pressure at the base of the lungs, which is nor-
mally about –2.5 mm Hg (relative to atmospheric) at the
start of inspiration, decreases to about –6 mm Hg. The lungs
are pulled into a more expanded position. The pressure in
the airway becomes slightly negative, and air flows into the
lungs. At the end of inspiration, the lung recoil begins to
pull the chest back to the expiratory position, where the
recoil pressures of the lungs and chest wall balance. The
pressure in the airway becomes slightly positive, and air
flows out of the lungs. Expiration during quiet breathing is
passive in the sense that no muscles that decrease intratho-
racic volume contract. However, some contraction of the
inspiratory muscles occurs in the early part of expiration.
This contraction exerts a braking action on the recoil forces
and slows expiration.
Strong inspiratory efforts reduce intrapleural pressure to
values as low as –30 mm Hg, producing correspondingly
greater degrees of lung inflation. When ventilation is
increased, the extent of lung deflation is also increased by
active contraction of expiratory muscles that decrease
intrathoracic volume.FIGURE 35–4
Pulmonary and systemic circulations.
Repre-
sentative areas of blood flow are labeled with corresponding blood
pressure (mm Hg).
(Modified from Comroe JH Jr.:
Physiology of Respiration,
2nd ed.
Year Book, 1974.)
FIGURE 35–5
Relationship between the bronchial and
pulmonary circulations.
The pulmonary artery supplies pulmonary
capillary network
A.
The bronchial artery supplies capillary networks
B, C,
and
D.
Blue-colored areas represent blood of low O
2
content.
(Reproduced with permission from Murray JF:
The Normal Lung.
Saunders, 1986.)
(^120) / 80
24
9
25
0
120
0
2 8
14
12 12
14
120
80
12 12
10
20
30
Pulmonary
artery
A
B C
D
Bronchial artery Bronchial vein Azygos vein
Broncho-
pulmonary
vein
Pulmonary
vein
Bronchopulmonary arterial anastomosis
FIGURE 35–6
Pressure in the alveoli and the plural space
relative to atmospheric pressure during inspiration and
expiration.
The dashed line indicates what the intrapleural pressure
would be in the absence of airway and tissue resistance; the actual
curve (solid line) is skewed to the left by the resistance. Volume of
breath during inspiration/expiration is graphed for comparison.
Pressure in
alveoli
Inspi-
ration
Expi-
ration + 2
- 1
0
0.6
0.4
0.2
0
01234
− 1
− 2
Intrapleural
pressure − 3
− 4
− 5
− 6
Volume of
breath
Volume
(L)
Pressure
(mm Hg)
Time (s)