For venous blood to be properly arterialized, the distribution of air and blood within the lung is automatically matched to ensure effective gas exchange across alveolar–capillary membranes. Arterialization comprises a series of interrelated processes that begin with the mechanical performance of the ventilatory apparatus—that is, the lungs and the chest wall, including the rib cage, diaphragm, and abdominal wall. The ventilatory apparatus is critical for replenishing fresh air to the lungs for gas exchange. Although the function of each component of the lung and of the chest bellows can be deranged by injury or disease, the design of the ventilatory apparatus provides for considerable reserve. As a result, mechanical derangements are usually quite severe by the time clinical symptoms appear or arterial blood-gas levels become abnormal.
Depending on the nature of the underlying disorder, assessment of the mechanical properties of the ventilatory apparatus provides several different types of information. In some instances, characterization of the mechanical abnormality provides insight into pathogenesis and affords a quantitative measure of severity. In others, once the nature of the mechanical disorder is understood, the mystery surrounding a life-threatening disorder in gas exchange may be dispelled. Finally, certain breathing patterns make sense only if the mechanical performance of the chest bellows is taken into account.
During breathing, the lungs and chest wall operate in unison. The lungs fill the chest cavity so that the visceral pleura are in contact with the parietal pleura of the chest wall. The two pleural surfaces are separated by only a thin liquid film, which provides the bond holding the lungs and chest wall together.
At the end of a normal exhalation when the respiratory muscles are at rest, the ventilatory apparatus is in a state of mechanical equilibrium. The pressure along the entire tracheobronchial tree from the airway opening to the alveoli is equal to atmospheric pressure. The tendency of the lung is to deflate, however, and lung elastic recoil is directed centripetally. This is counterbalanced by the elastic recoil of the chest wall, which is directed centrifugally to favor an increase in volume. These opposing forces generate a subatmospheric pleural pressure of about −5 cmH2O (Fig. 10-1A). The tendency for the lung to recoil inward and for the chest wall to recoil outward is illustrated by the observation that when the chest is opened at autopsy, the lungs collapse to a nearly airless state and the thorax expands.
Respiratory pressures during a breathing cycle. A. End expiration. B. During inspiration. C. End inspiration. Ppl, pleural pressure; PA, pressure in the alveoli; Pao, pressure at the airway opening.
Although it is conventional to consider pleural pressure as a single, mean value that reflects mechanical events within the entire ventilatory apparatus, this is clearly ...