The Amount Of AlveoIarVentiIation in Relation to Pulmonary Capillary Blood Flow—the V/QRatio—Determines the Adequacy of Pulmonary Gas Exchange

In the alveoli, gas exchange is accomplished by the close approximation of air and blood. Ideally, each of the millions of alveoli should receive air and blood in amounts that are optimal for gas exchange; that is, ventilation (V) and per­fusion (Q) should be matched.

Figure 47-5 shows schematic alveoli and capillaries with a variety of V/Q ratios. The alveolus in the center is ideal: it receives ventilation and blood flow with a V/Q ratio of 0.8. Venous blood arrives with a Pvo₂ and a PvCO₂ each of 46 mm Hg, is exposed to alveolar gas tensions (PAo₂ and PAco₂), and leaves with an end-capillary oxygen tension (Pcapo₂) of IOO mm Hg and a capillary carbon dioxide tension (Pcapco₂) of 40 mm Hg. The units toward the bottom have a low V/Q ratio and are relatively overperfused and under­ventilated. The Po₂ of blood leaving these units is low, and the Pco₂ is high. Low^r V/Q units occur frequently in lung disease because ventilation is reduced by airway obstruction or by localized stiffening of the lung by inflammatory processes. The alveoli toward the top of Figure 47-5 have a high V/Q ratio; ventilation is high in relation to blood flow. This can occur when pulmonary blood flow to part of the lung is reduced by vascular obstruction or by pulmonary hypotension. The blood leaving such units has a higher Po₂ and a lower Pco₂ than blood from the units with a V/Q ratio of 0.8.

Extending the concepts demonstrated in Figure 47-5 to the whole lung with its multitude of alveoli requires computer simulation and the investigation of the frequency distribution of V/Q ratios within the lung. In the normal animal the major­ity of alveoli have V/Q ratios close to 1, and the range of ratios is small (Figure 47-6).