Overview Supine distribution of pulmonary blood flow in lung is nearly uniform Standing distribution of pulmonary blood flow in lung is uneven due to effects of gravity Zone 1 PA ≥ Pa > PV PA = alveolar pressure Pa = arterial pressure PV = venous pressure PA ≥ Pa pulmonary capillaries may be compressed by high alveolar pressure reduces blood flow in normal lung, Pa is just high enough to prevent compression and to permit perfusion ↓ Pa → PA > Pa → pulmonary capillary compression and occlusion e.g., hemorrhage no zone 1 perfusion no gas exchange → ↑ physiologic dead space ↑ PA → PA > Pa → pulmonary capillary compression and occlusion e.g., positive pressure ventilation no zone 1 perfusion no gas exchange → ↑ physiologic dead space Zone 2 Pa > PA > PV PA > PV blood flow driven by difference between arterial pressure and alveolar pressure Zone 3 Pa > PV > PA PV > PA blood flow driven by difference between arterial pressure and venous pressure greatest number of pulmonary capillaries open highest blood flow