Figure 49-9 Schematic representation of the effects of gravity on the distribution of pulmonary blood flow in the lateral decubitus position. The vertical gradient in the lateral decubitus position is less than in the upright position. Consequently, there is less zone 1 and more zone 2 and 3 blood flow in the lateral decubitus position than in the upright position. Nevertheless, pulmonary blood flow increases with lung dependency and is greater in the dependent lung than in the nondependent lung. P_A , alveolar pressure; P_pa , pulmonary artery pressure; P_pv , pulmonary venous pressure. (From Benumof JL: Anesthesia for Thoracic Surgery. Philadelphia, WB Saunders, 1987.)