750
UNIT FIVE
thoracic cavity from the visceral pleura attached to the sur-
face of the lungs. The water molecules in this fl
uid greatly
attract the pleural membranes and each other, helping to
hold the moist surfaces of the pleural membranes tightly
together, much as a wet coverslip sticks to a microscope
slide. As a result of these factors, when the intercostal mus-
cles move the thoracic wall upward and outward, the pari-
etal pleura moves too, and the visceral pleura follows it. This
helps expand the lung in all directions.
Although the moist pleural membranes help expand the
lungs, the moist inner surfaces of the alveoli have the oppo-
site effect. Here the attraction of water molecules to each
other creates a force called
surface tension
that makes it dif-
F
cult to infl
ate the alveoli and may collapse them. Certain
alveolar cells, however, synthesize a mixture of lipoproteins
called
surfactant,
which is secreted continuously into alveo-
lar air spaces. Surfactant reduces the alveoli’s tendency to
collapse, especially when lung volumes are low, and makes
it easier for inspiratory efforts to infl ate the alveoli.
Table
19.2
summarizes the steps of inspiration.
Surfactant is particularly important in the minutes after birth, when
the newborn’s lungs inF
ate for the ±
rst time. Premature infants often
suffer respiratory distress syndrome because they do not produce
su²
cient surfactant. To help many of these newborns survive, physi-
cians inject synthetic surfactant into the tiny lungs through an endo-
tracheal tube. A ventilator machine especially geared to an infant’s
size assists breathing.
If a person needs to take a deeper than normal breath,
the diaphragm and external intercostal muscles contract
pressure inside increases, forcing air out into the atmosphere
(F g. 19.22
b
). The movement of air into and out of the lungs
occurs in much the same way.
If the pressure inside the lungs and alveoli (intra-alveolar
pressure) decreases, outside air will then be pushed into
the airways by atmospheric pressure. This is what happens
during normal inspiration, and it uses muscle F
bers in the
dome-shaped
diaphragm.
The diaphragm is just inferior to the lungs. It consists of
an anterior group of skeletal muscle F bers (costal F bers) that
originate from the ribs and sternum, and a posterior group
(crural F bers) that originate from the vertebrae. Both groups
of muscle F
bers are inserted on a tendinous central portion
of the diaphragm (reference plate 21).
The muscle fibers of the diaphragm are stimulated to
contract by impulses carried by the
phrenic nerves,
which are
associated with the cervical plexuses. When this happens, the
diaphragm moves downward, the thoracic cavity enlarges, and
the intra-alveolar pressure falls about 2 mm Hg below atmo-
spheric pressure. In response to this decreased pressure, air is
forced into the airways by atmospheric pressure
(f g. 19.23)
.
While the diaphragm is contracting and moving down-
ward, the
external
(
inspiratory
)
intercostal muscles
and cer-
tain thoracic muscles may be stimulated to contract. This
action raises the ribs and elevates the sternum, increasing
the size of the thoracic cavity even more. The intra-alveolar
pressure falls farther, and atmospheric pressure forces more
air into the airways.
Lung expansion in response to movements of the dia-
phragm and chest wall depends on movements of the pleu-
ral membranes. Any separation of the pleural membranes
decreases pressure in the intrapleural space, holding these
membranes together. In addition, only a thin F
lm of serous
fl uid separates the parietal pleura on the inner wall of the
FIGURE 19.22
Moving the plunger of a syringe causes air to
move (
a
) in or (
b
) out of the syringe. Air movements in and out of
the lungs occur in much the same way.
Atmospheric pressure
of 760 mm Hg on the
outside
Atmospheric
pressure
of 760 mm Hg
on the inside
Diaphragm
Air passageway
(a)
(b)
FIGURE 19.21
When the lungs are at rest, the pressure on the
inside of the lungs is equal to the pressure on the outside of the thorax.
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