448
Chapter 13
air fl ow. By defi nition, if there is no air fl ow,
P
alv
must equal
P
atm
(see equation 13–2). Because the lungs always have air in
them, the transmural pressure of the lungs (
P
tp
) must always
be positive and, therefore,
P
alv
>
P
ip
. At rest, when there is no
air fl ow and
P
alv
= 0,
P
ip
must be negative, providing the force
that keeps the lungs open and the chest wall in.
What are the forces that cause
P
ip
to be negative? The
fi rst, the
elastic recoil
of the lungs, is defi ned as the tendency
of an elastic structure to oppose stretching or distortion. Even
at rest, the lung contains air, and its natural tendency is to col-
lapse because of elastic recoil. The lungs are held open by the
positive
P
tp
, which, at rest, exactly opposes elastic recoil. The
chest wall also has elastic recoil, and, at rest, its natural ten-
dency is to expand.
At rest, all of these transmural pressures balance each
other out. It is clear that the subatmospheric (negative) intra-
pleural pressure (
P
ip
) is the essential factor keeping the lungs
Figure 13–9
Pressure differences involved in ventilation. Transpulmonary pressure
(
P
tp
=
P
alv
P
ip
) is a determinant of lung size. Intrapleural pressure
(
P
ip
) at rest is a balance between the tendency of the lung to collapse
and the tendency of the chest wall to expand.
P
cw
represents the
transmural pressure across the chest wall (
P
ip
P
atm
).
P
alv
P
atm
is the
driving pressure gradient for airfl ow in and out of the lungs. (The
volume of intrapleural fl uid is greatly exaggerated for visual clarity.)
P
tp
P
alv
P
a
lv
P
a
tm
Lung wall
Chest wall
Intrapleural fluid
Atmosphere
P
atm
P
ip
P
cw
P
atm
elastic structures—like balloons—and their volume, therefore,
depends on two factors. The fi rst is the difference in pressure
between the inside and outside of the lung, termed the
trans-
pulmonary pressure (
P
tp
).
The second is how stretchable
the lungs are, which determines how much they expand for a
given change in
P
tp
. The rest of this section and the next three
sections focus on transpulmonary pressure; stretchability will
be discussed later in the section on lung compliance.
The pressure inside the lungs is the air pressure inside the
alveoli (
P
alv
), and the pressure outside the lungs is the pressure
of the intrapleural fl uid surrounding the lungs (
P
ip
). Thus,
Transpulmonary pressure =
P
alv
P
ip
(13–3)
P
tp
=
P
alv
P
ip
Compare this equation to equation 13–2 (the equation that
describes air fl ow into or out of the lungs), as it will be essential
to distinguish these equations from each other (
Figure 13–9
).
Transpulmonary pressure is the
transmural pressure
that governs the static properties of the lungs.
Transmural
means “across a wall” and, by convention, is represented by
the pressure in the inside of the structure (
P
i
) minus the pres-
sure outside the structure (
P
O
). Infl ation of a balloon-like
structure like the lungs requires an increase in the transmural
pressure such that
P
i
increases relative to
P
O
.
Table 13–3
and Figure 13–9 show the major transmu-
ral pressures of the respiratory system. The transmural pres-
sure acting on the lungs (
P
tp
) is
P
alv
P
ip
and on the chest
wall (
P
cw
) is
P
ip
P
atm
. The muscles of the chest wall and the
diaphragm contract and cause the chest wall to expand during
inspiration. As the chest wall expands,
P
ip
decreases according
to Boyle’s law.
P
tp
becomes more positive as a result and the
lungs expand. As this occurs,
P
alv
becomes more negative com-
pared to
P
atm
(again due to Boyle’s law), and air fl ows inward
(inspiration, equation 13–2). Therefore, the transmural pres-
sure of the lungs (
P
tp
) is increased to fi ll it with air by actively
decreasing the pressure surrounding the lungs (
P
ip
) relative to
the pressure inside the lungs (
P
alv
). When the respiratory mus-
cles relax, elastic recoil of the lungs drives passive expiration
back to the starting point.
How Is a Stable Balance Achieved
Between Breaths?
Figure 13–10
illustrates the transmural pressures of the respi-
ratory system at rest—that is, at the end of an unforced expira-
tion when the respiratory muscles are relaxed and there is no
Table 13–3
Two Important Transmural Pressures of the Respiratory System
Transmural Pressure
P
i
P
O
Value at Rest
Explanatory Notes
Transpulmonary (
P
tp
)
P
alv
P
ip
0 – [–4] = 4 mmHg
Pressure difference holding lungs open (opposes
inward elastic recoil of the lung)
Chest wall (
P
cw
)
P
ip
P
atm
–4 – 0 = –4 mmHg
Pressure difference holding chest wall in (opposes
outward elastic recoil of the chest wall)
P
i
is pressure inside the structure, and
P
O
is pressure surrounding the structure.
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