Figure 17-4 Distribution of ventilation and blood flow (left-hand vertical axis) and the ventilation-perfusion ratio (V̇A/, right-hand vertical axis) in normal upright lung. Both blood flow and ventilation are expressed in liters per minute per percentage of alveolar volume and have been drawn as smoothed-out linear functions of vertical height. The closed circles mark the V̇A/ ratios of horizontal lung slices (three of which are shown in Fig. 17-5 ). A cardiac output of 6 L/min and a total minute ventilation of 5.1 L/min were assumed. (Redrawn with modification from West JB: Ventilation/Blood Flow and Gas Exchange, 4th ed. Oxford, Blackwell Scientific, 1970.)