Pulmonary blood flow vs. gas volume at various perfusion pressures in rabbit lung

To obtain a detailed description of the dependence of pulmonary blood flow on changes in lung volume, we perfused isolated rabbit lungs with homologous blood at 37 degrees C while controlling vascular pressures during lung deflation. We set pulmonary arterial pressure (Ppa) and pulmonary venous pressure (Ppv) to constant values relative to alveolar pressure (Palv) to keep the effective driving pressure for flow constant during lung deflation from total lung capacity (TLC) to 25% TLC. The shapes of the flow vs. lung volume curves were dependent on the levels of Ppa-Palv and Ppv-Palv at which they were obtained. When Ppv greater than Palv throughout the lung (zone 3 conditions), flow increased as the lungs were deflated from TLC, independent of the level of Ppa-Palv. When Ppv less than Palv (zone 2 conditions) and Ppa-Palv was moderately high, flow increased as the lungs were deflated from 100 to approximately 50% TLC, then decreased at lower lung volumes. When Ppa - Palv was less than 10 cmH2O in zone 2 conditions, flow decreased monotonically during deflation from TLC. We concluded that the dependence of blood flow on lung volume is complex, which may be a reflection of the nonlinear pressure-diameter properties of pulmonary vessels.