Figure 17-1 Schematic diagram showing the distribution of blood flow in the upright lung. In zone 1, alveolar pressure (PA) exceeds pulmonary artery pressure (Ppa), and no flow occurs because the intra-alveolar vessels are collapsed by the compressing alveolar pressure. In zone 2, Ppa exceeds PA, but PA exceeds pulmonary venous pressure (Ppv). Flow in zone 2 is determined by the Ppa-PA difference (Ppa - PA) and has been likened to an upstream river waterfall over a dam. Because Ppa increases down zone 2 whereas PA remains constant, perfusion pressure increases, and flow steadily increases down the zone. In zone 3, Ppv exceeds PA, and flow is determined by the Ppa-Ppv difference (Ppa - Ppv), which is constant down this portion of the lung. However, transmural pressure across the wall of the vessel increases down this zone, so the caliber of the vessels increases (resistance decreases), and therefore flow increases. Finally, in zone 4, pulmonary interstitial pressure becomes positive and exceeds both Ppv and PA. Consequently, flow in zone 4 is determined by the Ppa-interstitial pressure difference (Ppa - PISF). (Redrawn with modification from West JB: Ventilation/Blood Flow and Gas Exchange, 4th ed. Oxford, Blackwell Scientific, 1970.)