Porth's Essentials of Pathophysiology, 4e

522

Respiratory Function

U N I T 6

Airflow in

Airflow out

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.

Pressure

Pressure

Inspiration

Expiration

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

Increased vertical diameter

Increased A–P diameter

External intercostals contracted

Internal intercostals contracted

Diaphragm relaxed

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.

Diaphragmatic contraction

Abdominals contracted Expiration

A

B

Inspiration