Respiratory Physiology
449
partially expanded between breaths. An extremely important
question is: What is the reason for a subatmospheric (nega-
tive)
P
ip
?
As the lungs tend to collapse and the thoracic wall tends
to expand, they move ever so slightly away from each other.
This causes an infi nitesimal enlargement of the fl
uid-fi lled
intrapleural space between them. But fl uid cannot expand
the way air can, so even this tiny enlargement of the intra-
pleural space—so small that the pleural surfaces still remain
in contact with each other—decreases the intrapleural pres-
sure below atmospheric pressure. In this way, the elastic recoil
of both the lungs and chest wall creates the subatmospheric
intrapleural pressure that keeps them from moving apart more
than a very tiny amount. Again, imagine trying to pull apart
two glass slides that have a drop of water between them. The
fl uid pressure generated between the slides will be lower than
atmospheric pressure.
The importance of the transpulmonary pressure in
achieving this stable balance can be seen when, during sur-
gery or trauma, the chest wall is pierced without damaging
the lung. Atmospheric air rushes through the wound into the
intrapleural space, a phenomenon called
pneumothorax,
and
the intrapleural pressure goes from –4 mmHg to 0 mmHg.
The transpulmonary pressure acting to hold the lung open is
thus eliminated, and the lung collapses. At the same time, the
chest wall moves outward because its elastic recoil is also no
longer opposed. The thoracic cavity is divided into right and
left sides by a structure called the
mediastinum,
so the pneu-
mothorax is often unilateral (
Figure 13–11
).
Inspiration
Figures 13–12
and
13–13
summarize the events that occur
during normal inspiration at rest. Inspiration is initiated by
the neurally induced contraction of the diaphragm and the
Chest
wall
Intrapleural
space
Lung elastic
recoil
P
alv
0
Chest wall
elastic recoil
P
atm
= 0
P
atm
0
P
ip
– 4
P
tp
P
cw
Figure 13–10
Alveolar (
P
alv
), intrapleural (
P
ip
), transpulmonary (
P
tp
), and trans-
chest-wall (
P
cw
) pressures (mmHg) at the end of an unforced
expiration—that is, between breaths when there is no air fl ow. The
transpulmonary pressure (
P
alv
P
ip
) exactly opposes the elastic recoil
of the lung, and the lung volume remains stable. Similarly, trans-
chest wall pressure (
P
ip
P
atm
) is balanced by the outward elastic
recoil of the chest wall. Notice that transmural pressures are the
pressure inside the wall minus the pressure outside the wall. (The
volume of intrapleural fl uid is greatly exaggerated for clarity.)
Figure 13–11
Pneumothorax due to trauma (motor vehicle collision). Arrows
show outline of collapsed left lung. Right lung (shown on left) is not
affected. Notice free air between collapsed left lung and chest wall.
Air flows into alveoli
P
alv
becomes subatmospheric
Lungs
Expand
Transpulmonary pressure
P
ip
becomes more subatmospheric
Diaphragm and inspiratory intercostals contract
Thorax
Expands
Figure 13–12
Sequence of events during inspiration. Figure 13–13 illustrates these
events quantitatively.
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