Chapter 22
The Respiratory System
817
22
Intrapleural Pressure (
P
ip
)
Te pressure in the pleural cavity, the
intrapleural pressure (
P
ip
)
,
also fluctuates with breathing phases, but is always about 4 mm Hg
less than
P
pul
. Tat is,
P
ip
is
always
negative relative to
P
pul
.
What causes this negative intrapleural pressure? ±o answer
this question, let’s examine the forces that exist in the thorax.
First of all, we know there are opposing forces. ±wo forces act to
pull the lungs (visceral pleura) away from the thorax wall (pari-
etal pleura) and cause the lungs to collapse:
The lungs’ natural tendency to recoil.
Because of their elas-
ticity, lungs always assume the smallest size possible.
The surface tension of the alveolar fluid.
Te molecules of
the fluid lining the alveoli attract each other. Tis produces
surface tension
that constantly acts to draw the alveoli to their
smallest possible dimension.
However, these lung-collapsing forces are opposed by the
natural elasticity of the chest wall, a force that tends to pull the
thorax outward and enlarge the lungs. So which force wins? In
a healthy person, the answer is neither, because of the strong
adhesive force between the parietal and visceral pleurae. Pleural
fluid secures the pleurae together in the same way a drop of
water holds two glass slides together. Te pleurae slide from
side to side easily, but they remain closely apposed, and separat-
ing them requires extreme force. Te net result of the dynamic
interplay between these forces is a negative
P
ip
.
Te amount of pleural fluid in the pleural cavity must remain
minimal to maintain a negative
P
ip
. Te pleural fluid is actively
pumped out of the pleural cavity into the lymphatics continu-
ously. If it wasn’t, fluid would accumulate in the intrapleural
space (remember, fluids move from high to low pressure), pro-
ducing a positive pressure in the pleural cavity.
We cannot overemphasize the importance of negative pressure
in the intrapleural space and the tight coupling of the lungs to the
thorax wall. Any condition that equalizes
P
ip
with the intrapulmo-
nary (or atmospheric) pressure causes
immediate lung collapse
. It
is the
transpulmonary pressure
—the difference between the in-
trapulmonary and intrapleural pressures (
P
pul
P
ip
)—that keeps
the air spaces of the lungs open or, phrased another way, keeps the
lungs from collapsing. Moreover,
the size of the transpulmonary
pressure determines the size of the lungs
at any time—the greater
the transpulmonary pressure, the larger the lungs.
Homeostatic Imbalance
22.7
Atelectasis
(at
0
ĕ-lik
9
tah-sis), or lung collapse, occurs when a
bronchiole becomes plugged (as may follow pneumonia). Its
associated alveoli then absorb all of their air and collapse. Atel-
ectasis can also occur when air enters the pleural cavity either
through a chest wound or a rupture of the visceral pleura, which
allows air from the respiratory tract to enter the pleural cavity.
Te presence of air in the pleural cavity is referred to as a
pneumothorax
(nu
0
mo-tho
9
raks; “air thorax”), and is reversed
by drawing air out of the intrapleural space with chest tubes.
Tis procedure allows the pleurae to heal and the lung to rein-
flate and resume normal function. Note that because the lungs
are in separate cavities, one lung can collapse without interfer-
ing with the other.
Pulmonary Ventilation
Relate Boyle’s law to events of inspiration and expiration.
Explain the relative roles of the respiratory muscles and
lung elasticity in producing the volume changes that cause
air to flow into and out of the lungs.
Pulmonary ventilation, consisting of inspiration and expiration,
is a mechanical process that depends on volume changes in the
thoracic cavity. A rule to keep in mind throughout the following
discussion is that
volume changes
lead to
pressure changes
, and
pressure changes lead to the
flow of gases
to equalize the pressure.
Boyle’s law
gives the relationship between the pressure and
volume of a gas: At constant temperature, the pressure of a gas
varies inversely with its volume. Tat is,
P
1
V
1
5
P
2
V
2
where
P
is the pressure of the gas,
V
is its volume, and subscripts
1 and 2 represent the initial and resulting conditions respectively.
Gases always
fill
their container. Consequently, in a large con-
tainer, the molecules in a given amount of gas will be far apart
and the pressure will be low. But if the volume of the container
is reduced, the gas molecules will be forced closer together and
the pressure will rise.
Atmospheric pressure (
P
atm
)
0 mm Hg (760 mm Hg)
Intrapleural
pressure (
P
ip
)
-
4 mm Hg
(756 mm Hg)
Transpulmonary
pressure
4 mm Hg
(the difference
between 0 mm Hg
and
-
4 mm Hg)
Thoracic wall
Diaphragm
Lung
Intrapulmonary
pressure (
P
pul
)
0 mm Hg
(760 mm Hg)
Parietal pleura
Pleural cavity
Visceral pleura
0
-
4
Figure 22.12
Intrapulmonary and intrapleural pressure
relationships.
Values shown are pressures relative to atmospheric
pressure (760 mm Hg at sea level). Absolute pressures are given in
parentheses. These pressures are at the end of a normal expiration.
For illustration, the size of the pleural cavity has been greatly
exaggerated.
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