When the diaphragm lowers, the abdominal organs infe-
rior to it are compressed. As the diaphragm and the external
intercostal muscles relax following inspiration, the elastic
tissues cause the lungs to recoil, and they return to their
original shapes. Similarly, elastic tissues cause abdominal
organs to spring back into their previous shapes, pushing
the diaphragm upward. At the same time, surface tension
that develops between the moist surfaces of the alveolar lin-
ings shrinks alveoli. Each of these factors increases the intra-
alveolar pressure about 1 mm Hg above atmospheric pressure,
so the air inside the lungs is forced out through the respiratory
passages. Normal resting expiration occurs without the con-
traction of muscles. Thus, it is a passive process.
The recoil of elastic F
bers in lung tissues reduces pres-
sure in the pleural cavity. Consequently, the pressure
between the pleural membranes (intrapleural pressure) is
usually about 4 mm Hg less than atmospheric pressure.
The visceral and parietal pleural membranes are held closely together
because of the low intrapleural pressure, and no significant space
normally separates them in the pleural cavity. However, if the tho-
racic wall is punctured, atmospheric air may enter the pleural cavity
and create a substantial space between the membranes. This condi-
can collapse the lung on the aF
because of its elasticity.
Pneumothorax may be treated by covering the chest wound
with an impermeable bandage, passing a tube (chest tube) through
the thoracic wall into the pleural cavity, and applying suction to the
tube. The suction reestablishes negative pressure in the cavity and
the collapsed lung expands.
more forcefully. Additional muscles, such as the pectoralis
minors and sternocleidomastoids, can also be used to pull
the thoracic cage farther upward and outward, enlarging the
thoracic cavity and decreasing intra-alveolar pressure even
(f g. 19.24)
The ease with which the lungs can expand as a result of
pressure changes during breathing is called
tensibility). In a normal lung, compliance decreases as lung
volume increases, because an inﬂ
ated lung is more difF
to expand than a lung at rest. Conditions that obstruct air
passages, destroy lung tissue, or impede lung expansion in
other ways also decrease compliance.
The forces responsible for normal resting expiration come
of lung tissues and from surface tension.
The lungs contain a considerable amount of elastic tissue,
which stretches as the lungs expand during inspiration.
(760 mm Hg)
(758 mm Hg)
(760 mm Hg)
) Prior to inspiration, the intra-alveolar pressure is 760 mm Hg. (
) The intra-alveolar pressure decreases to
about 758 mm Hg as the thoracic cavity enlarges, and atmospheric pressure forces air into the airways.
Major Events in Inspiration
1. Nerve impulses travel on phrenic nerves to muscle ²
bers in the
diaphragm, contracting them.
2. As the dome-shaped diaphragm moves downward, the thoracic
3. At the same time, the external intercostal muscles may contract,
raising the ribs and expanding the thoracic cavity further.
4. The intra-alveolar pressure decreases.
5. Atmospheric pressure, greater on the outside, forces air into the
respiratory tract through the air passages.
6. The lungs ²
ll with air.