522
U N I T 6
Respiratory Function
to side and from front to back. The external intercostal
muscles receive their innervation from nerves that exit
the central nervous system (CNS) at the thoracic level of
the spinal cord. Paralysis of these muscles usually does
not have a serious effect on respiration because of the
effectiveness of the diaphragm.
The accessory muscles of inspiration include the sca-
lene muscles and the sternocleidomastoid muscles. The
scalene muscles elevate the first two ribs, and the sterno-
cleidomastoid muscles raise the sternum to increase the
size of the chest cavity. These muscles contribute little to
quiet breathing but contract vigorously during exercise.
For the accessory muscles to assist in ventilation, they
must be stabilized in some way. Persons with bronchial
asthma often brace their arms against a firm object during
an attack as a means of stabilizing their shoulders so that
the attached accessory muscles can exert their full effect
on ventilation. The head commonly is bent backward so
that the scalene and sternocleidomastoid muscles can ele-
vate the ribs more effectively. Other muscles that play a
minor role in inspiration are the alae nasi, which produce
flaring of the nostrils during obstructed breathing.
Expiration is largely passive. It occurs as the elastic
components of the chest wall and lung structures that
were stretched during inspiration recoil, causing air to
leave the lungs as the intrathoracic pressure increases.
When needed, the abdominal and the internal inter-
costal muscles can be used to increase expiratory effort
(see Fig. 21-11B). The increase in intra-abdominal
pressure that accompanies the forceful contraction of
the abdominal muscles pushes the diaphragm upward
and results in an increase in intrathoracic pressure. The
internal intercostals pull the ribs downward and inward,
assisting in exhalation.
Lung Compliance
Lung compliance refers to the ease with which the lungs
can be inflated. Compliance can be appreciated by com-
paring the ease of blowing up a balloon that has been
previously inflated with a new balloon that is stiff and
noncompliant. Specifically, lung compliance is a mea-
sure of the change in lung volume that occurs with a
change in intrapulmonary pressure.
Lung compliance is determined by the elastic prop-
erties of the lung and alveolar surface tension. It also
depends on the compliance of the thoracic or chest cage.
It is diminished in conditions that reduce the natural
elastic properties of the lung, increase the surface tension
in the alveoli, or impair the flexibility of the chest cage.
Elastic Properties.
The elastic properties of the lung
involve at least three basic components: distensibility,
stiffness, and elastic recoil.
Distensibility
is the ease with
which the lungs can be inflated
Stiffness
is defined as the
resistance to stretch or inflation. Lung tissue is made up
of elastin and collagen fibers. The elastin fibers are easily
stretched and increase the ease of lung inflation, whereas
Airflow in
Airflow out
Inspiration
Expiration
Pressure
Pressure
FIGURE 21-10.
Movement of the
diaphragm and changes in chest
volume and pressure during inspiration
and expiration. During inspiration,
contraction of the diaphragm and
expansion of the chest cavity produce
a decrease in intrathoracic pressure,
causing air to move into the lungs.
During expiration, relaxation of the
diaphragm and chest cavity produces
an increase in intrathoracic pressure,
causing air to move out of the lungs.
External
intercostals
contracted
Increased A–P
diameter
Increased vertical
diameter
Diaphragmatic
contraction
Internal
intercostals
contracted
Abdominals
contracted
Expiration
Inspiration
Diaphragm
relaxed
A
B
FIGURE 21-11.
Expansion and contraction
of the chest cage during expiration and
inspiration, demonstrating especially
diaphragmatic contraction, elevation of the
rib cage, and function of the
(A)
external
and
(B)
internal intercostals.
