Respiratory Physiology
451
farther away from the lung surface. The intrapleural fl
uid pres-
sure
the
re
fo
re
become
s
even
mo
re
suba
tmo
sphe
r
ic
than
i
t
was between breaths. This decrease in intrapleural pressure
increases
the transpulmonary pressure. Therefore, the force
acting to expand the lungs—the transpulmonary pressure—is
now greater than the elastic recoil exerted by the lungs at this
moment, and so the lungs expand further. Note in Figure 13–13
that, by the end of inspiration, equilibrium
across the lungs
is
once again established because the more infl ated lungs exert a
greater elastic recoil, which equals the increased transpulmo-
nary pressure. In other words, lung volume is stable whenever
transpulmonary pressure is balanced by the elastic recoil of
the lungs (that is, at the end of both inspiration and expiration
when there is no air fl ow).
Thus, when contraction of the inspiratory muscles actively
increases the thoracic dimensions, the lungs are passively forced
to enlarge. The enlargement of the lungs causes an increase in
the sizes of the alveoli throughout the lungs. By Boyle’s law,
the pressure within the alveoli decreases to less than atmo-
spheric (see Figure 13–13). This produces the difference in
pressure (
P
alv
<
P
atm
) that causes a bulk fl ow of air from the
atmosphere through the airways into the alveoli. By the end
of the inspiration, the pressure in the alveoli again equals
atmospheric pressure because of this additional air, and air
fl ow ceases.
Expiration
Figure 13–13 and
Figure
13–15
summarize the sequence of
events that occur during expiration. At the end of inspiration,
the nerves to the diaphragm and inspiratory intercostal mus-
cles decrease their fi ring, and so these muscles relax. The dia-
phragm and chest wall are no longer actively pulled outward
by the muscle contractions, and so they start to recoil inward
to their original smaller dimensions that existed between
breaths. This immediately makes the intrapleural pressure less
subatmospheric thereby
decreasing
the transpulmonary pres-
sure. Therefore, the transpulmonary pressure acting to expand
the lungs is now smaller than the elastic recoil exerted by the
more expanded lungs, and the lungs passively recoil to their
original dimensions (see Figure 13–14).
As the lungs become smaller, air in the alveoli becomes
temporarily compressed so that, by Boyle’s law, alveolar pressure
exceeds atmospheric pressure (see Figure 13–13). Therefore,
air fl ows from the alveoli through the airways out into the
atmosphere. Thus, expiration at rest is passive, depending only
upon the relaxation of the inspiratory muscles and the elastic
recoil of the stretched lungs.
Under certain conditions, such as during exercise, expira-
tion of larger volumes is achieved by contraction of a different
Figure 13–14
X-ray of chest at full inspiration. The dashed white line is an outline
of the lungs in full expiration.
Air flows out of lungs
P
alv
becomes greater than
P
atm
Air in alveoli becomes compressed
Lungs
Recoil toward preinspiration size
Transpulmonary pressure moves
back toward preinspiration value
P
ip
moves back toward preinspiration value
Diaphragm and inspiratory intercostals stop contracting
Chest wall
Recoils inward
Figure 13–15
Sequence of events during expiration. Figure 13–13 illustrates these
events quantitatively.
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