Figure 17-5 Ventilation-perfusion ratio (V̇A/) and the regional composition of alveolar gas. Values for regional flow (), ventilation (V̇A), PO₂ , and PCO₂ were derived from Figure 17-4 . PN₂ was obtained by what remains from total gas pressure (which, including water vapor, equals 760 mm Hg). The volumes [Vol. (%)] of the three lung slices are also shown. When compared with the top of the lung, the bottom of the lung has a low V̇A/ ratio and is relatively hypoxic and hypercapnic. (Redrawn from West JB: Regional differences in gas exchange in the lung of erect man. J Appl Physiol 17:893, 1962.)