Chapter 22
The Respiratory System
819
22
pressure causes air to rush into the box from the atmosphere,
because gases always flow down their pressure gradients.
Te same thing happens during normal quiet inspiration, when
the
inspiratory muscles
—the diaphragm and external intercostal
muscles—are activated. Here’s how quiet inspiration works:
Action of the diaphragm.
When the dome-shaped dia-
phragm contracts, it moves inferiorly and flattens out
(
Figure 22.13
, top). As a result, the superior-inferior di-
mension (height) of the thoracic cavity increases.
Action of the intercostal muscles.
When the external inter-
costal muscles contract, they liF the rib cage and pull the
sternum superiorly (±igure 22.13, top). Because the ribs
curve downward as well as forward around the chest wall,
the broadest lateral and anteroposterior dimensions of the
rib cage are normally directed obliquely downward. But
when the ribs are raised and drawn together, they swing out-
ward, expanding the diameter of the thorax both laterally
and in the anteroposterior plane. Tis is much like the action
that occurs when a curved bucket handle is raised—it moves
outward as it moves upward.
Although these actions expand the thoracic dimensions by
only a few millimeters along each plane, this is enough to in-
crease thoracic volume by almost 500 ml—the usual volume
of air that enters the lungs during a normal quiet inspiration.
Of the two types of inspiratory muscles, the diaphragm is far
more important in producing these volume changes that lead to
normal quiet inspiration.
As the thoracic dimensions increase during inspiration, the
lungs are stretched and the intrapulmonary volume increases. As
a result,
P
pul
drops about 1 mm Hg relative to
P
atm
. Anytime the
intrapulmonary pressure is less than the atmospheric pressure
(
P
pul
,
P
atm
), air rushes into the lungs along the pressure gradient.
Inspiration ends when
P
pul
5
P
atm
. During the same period,
P
ip
declines to about
2
6 mm Hg relative to
P
atm
(Figure 22.14)
.
During the
deep
or
forced inspirations
that occur during vigor-
ous exercise and in some chronic obstructive pulmonary diseases,
accessory muscles further increase thoracic volume. Several mus-
cles, including the scalenes and sternocleidomastoid muscles of the
neck and the pectoralis minor of the chest, raise the ribs even more
than during quiet inspiration. Additionally, the back extends as the
erector spinae muscles straighten the thoracic curvature.
Expiration
In healthy individuals, quiet expiration is a passive process that
depends more on lung elasticity than on muscle contraction. As
the inspiratory muscles relax and resume their resting length,
the rib cage descends and the lungs recoil (±igure 22.13, bot-
tom). As a result, both the thoracic and intrapulmonary vol-
umes decrease. Tis volume decrease compresses the alveoli,
and
P
pul
rises to about 1 mm Hg above atmospheric pressure
(±igure 22.14). When
P
pul
.
P
atm
, the pressure gradient forces
gases to flow out of the lungs.
Forced expiration
is an active process produced by con-
tracting abdominal wall muscles, primarily the oblique and
transversus muscles. Tese contractions (1) increase the intra-
abdominal pressure, which forces the abdominal organs supe-
riorly against the diaphragm, and (2) depress the rib cage. Te
internal intercostal muscles also help depress the rib cage and
decrease thoracic volume.
²o precisely regulate air flow from the lungs, it is necessary
to control the accessory muscles of expiration. ±or instance, the
ability of a trained vocalist to hold a musical note depends on
the coordinated activity of several muscles normally used in
forced expiration.
Pressure relative to
atmospheric pressure (mm Hg)
–8
–6
–4
–2
+2
Volume (L)
0.5
0
5 seconds elapsed
Volume of breath
Intrapulmonary
pressure
Expiration
Intrapleural
pressure
Trans-
pulmonary
pressure
Inspiration
Intrapulmonary pressure.
Pressure inside lung
decreases as lung volume
increases during
inspiration; pressure
increases during expiration.
Intrapleural pressure.
Pleural cavity pressure
becomes more negative as
chest wall expands during
inspiration. Returns to initial
value as chest wall recoils.
Volume of breath.
During
each breath, the pressure
gradients move 0.5 liter of
air into and out of the lungs.
0
Figure 22.14
Changes in intrapulmonary and intrapleural pressures during
inspiration and expiration.
Notice that normal atmospheric pressure (760 mm Hg) is
given a value of 0 on the scale.
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