Mechanics of Pulmonary Ventilation Muscles That Cause Lung Expansion and Contraction The lungs can be expanded and contracted in two ways: (1) by downward and
upward movement of the iaphragm to lengthen or shorten the chest cavity, and (2) by elevation and
depression of the ribs to increase and decrease the anteroposterior diameter of the chest cavity. Figure
37-1 shows these two methods. Normal quiet breathing is accomplished almost entirely by the first
method, that is, by movement of the diaphragm. During inspiration, contraction of the diaphragm pulls
the lower surfaces of the lungs downward. The most important muscles that raise the rib cage are the
external intercostals, but others that help are the (1) sternocleidomastoid muscles, which lift upward
on the sternum; (2) anterior serrati, which lift many of the ribs; and (3) scaleni, which lift the first two
ribs. The muscles that pull the rib cage downward during expiration are mainly the (1) abdominal recti, which have the powerful effect of pulling downward on the lower ribs at the same time that they
and other abdominal muscles also compress the abdominal contents upward against the diaphragm,
and (2) internal intercostals. Figure 37-1 also shows the mechanism by which the external and internal
intercostals act to cause inspirationand expiration. To the left, the ribs during expiration are angled
downward, and the external intercostals are elongated forward and downward. As they contract, they
pull the upper ribs forward in relation to the lower ribs, and this causes leverage on the ribs to raise
them upward, thereby causing inspiration. The internal intercostals function exactly in the opposite
manner, functioning as expiratory muscles because they angle between the ribs in the opposite
direction and cause opposite leverage.
