Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

has a water vapor pressure of 47 mm Hg, which must be included in the sum of the total pressure of the gases.

outward on the lungs to increase the negative pressure so air can move into the lungs; and during expiration, the pressures are reversed causing air to move out of the lung. Although the intrapleural pressure of the inflated lung is always negative in relation to alveolar pressure, it may become positive in relation to atmospheric pres- sure (e.g., as during forced expiration and coughing). The intrathoracic pressure is the pressure in the tho- racic cavity. It is essentially equal to intrapleural pres- sure and is the pressure to which the lungs, heart, and great vessels are exposed. Forced expiration against a closed glottis (Valsalva maneuver) compresses the air in the thoracic cavity and produces marked increases in both the intrathoracic and intrapleural pressures. Chest Cage and Respiratory Muscles The chest cage is a closed compartment bounded on the top by the neck muscles and at the bottom by the dia- phragm. The outer walls of the chest cage are formed by 12 pairs of ribs, the sternum, the thoracic vertebrae, and the intercostal muscles that lie between the ribs. The lungs and major airways share the inner chest cavity with the heart, great vessels, and esophagus. Mechanically, ventilation or the act of breathing depends on the fact that the chest cavity is a closed compartment whose only opening to the exterior is the trachea. Air moves between the atmosphere and the lungs because of a pressure difference or gradient. According to the laws of physics, the pressure of a gas varies inversely with the volume of its container, with the pressure of the same quantity of gas in a smaller container being greater than that in a larger container. The movement of gases is always from the container with the greater pressure to the one with the lesser pressure. The chest cavity can be likened to a volume container in which the pressure becomes more negative during inspiration as the chest cavity expands, and becomes more positive during expi- ration as the chest cage contracts. Because of the change in size and pressure of the chest cage, air moves into the lungs during inspiration and out of the lungs during expiration (Fig. 21-10). The diaphragm is the principal muscle of inspiration. When the diaphragm contracts, the abdominal contents are forced downward and the chest expands from top to bottom (see Fig. 21-10). During normal levels of inspiration, the diaphragm moves approximately 1 cm, but this can be increased to 10 cm on forced inspira- tion. The diaphragm is innervated by the phrenic nerve roots, which arise from the cervical level of the spinal cord, mainly from C4 but also from C3 and C5. Persons with spinal cord injury above this level require mechani- cal ventilation. Paralysis of one side of the diaphragm causes the chest to move up on that side rather than down during inspiration because of the negative pres- sure in the chest. This is called paradoxical movement . The external intercostal muscles, which aid in inspi- ration, connect to the adjacent ribs and slope downward and forward (Fig. 21-11). When they contract, they raise the ribs and rotate them slightly so that the ster- num is pushed forward, enlarging the chest from side

Pulmonary Ventilation Ventilation refers to the exchange of gases within the respiratory system. There are two types of ventilation: pulmonary and alveolar. Pulmonary ventilation refers to the total exchange of gases between the atmosphere and the lungs, and alveolar ventilation to the transfer of gases within the gas exchange portion of the lungs. Pulmonary ventilation relies on a system of open airways and a change in pressure that is created as the respiratory muscles change the size of the chest cage. The degree to which the lungs inflate and deflate depends on the movement of the chest cage and pressures created by respiratory muscles, the resistance that the air encoun- ters as it moves through the airways, and the compli- ance or ease with which the lungs can be inflated. Respiratory Pressures The pressure inside the airways and alveoli of the lungs is called the intrapulmonary pressure or alveolar pressure. The gases in the lungs are in communication with atmospheric air pressure (Fig. 21-9). When the glottis is open and air is not moving into or out of the lungs, as occurs just before inspiration or expiration, the intrapulmonary pressure is zero or equal to atmo- spheric pressure. The pressure in the pleural cavity is called the intra- pleural pressure. The intrapleural pressure is always negative in relation to alveolar pressure in the normally inflated lung. The lungs are elastic structures that would collapse and expel all their air were it not for the nega- tive intrapleural pressure (normally about −4 mm Hg between breaths) that holds them against the chest wall. During inspiration, expansion of the chest cage pulls

Intrapleural pressure

Airway pressure

Intra-alveolar pressure

Intrathoracic pressure

FIGURE 21-9. Partitioning of respiratory pressures.