This study investigates the distribution of blood flow in an isolated dog lung, relating it to changes in vascular and alveolar pressures. The left lung was removed, ventilated with negative pressure, and perfused with venous blood. The pulmonary arterial, venous, and alveolar pressures were varied, and the distribution of blood flow was measured using radioactive xenon. Key findings include: 1. **Flow Distribution**: Under normal conditions, no blood flow occurs above the level where alveolar pressure equals arterial pressure. Below this level, blood flow increases linearly down the lung when venous pressure is low. Raising venous pressure makes the flow distribution more uniform below the level where venous and alveolar pressures equalize. 2. **Mechanical Effects**: The flow distribution can be explained by the mechanical effects of pressure inside and outside the blood vessels, behaving like Starling resistance. This means that when the pressure inside the vessels is higher than the pressure outside, the vessels collapse; when the pressure inside is lower, the vessels dilate. 3. **Clinical Relevance**: The study simulates the flow distributions found in various physiological and diseased states in humans, providing insights into how changes in pressure affect blood flow distribution in the lung. 4. **Methodology**: The study used a detailed experimental setup, including a Lucite box for lung suspension, a perfusing circuit, and scintillation counters to measure blood flow distribution. Various techniques, such as injecting radiographic contrast material and Evans blue solution, were also used to validate the results. 5. **Discussion**: The observed distribution of blood flow in the lung is divided into three zones based on the heights of pulmonary arterial, alveolar, and venous pressures. Zone 1 has no blood flow due to alveolar pressure, Zone 2 shows a linear increase in flow with distance, and Zone 3 also shows a linear increase but at a potentially slower rate. The pressure outside the distensible resistive vessels in Zone 3 is primarily alveolar pressure. Overall, the study provides a comprehensive understanding of how pressure changes affect blood flow distribution in the lung, which has implications for both physiological and pathological conditions.This study investigates the distribution of blood flow in an isolated dog lung, relating it to changes in vascular and alveolar pressures. The left lung was removed, ventilated with negative pressure, and perfused with venous blood. The pulmonary arterial, venous, and alveolar pressures were varied, and the distribution of blood flow was measured using radioactive xenon. Key findings include: 1. **Flow Distribution**: Under normal conditions, no blood flow occurs above the level where alveolar pressure equals arterial pressure. Below this level, blood flow increases linearly down the lung when venous pressure is low. Raising venous pressure makes the flow distribution more uniform below the level where venous and alveolar pressures equalize. 2. **Mechanical Effects**: The flow distribution can be explained by the mechanical effects of pressure inside and outside the blood vessels, behaving like Starling resistance. This means that when the pressure inside the vessels is higher than the pressure outside, the vessels collapse; when the pressure inside is lower, the vessels dilate. 3. **Clinical Relevance**: The study simulates the flow distributions found in various physiological and diseased states in humans, providing insights into how changes in pressure affect blood flow distribution in the lung. 4. **Methodology**: The study used a detailed experimental setup, including a Lucite box for lung suspension, a perfusing circuit, and scintillation counters to measure blood flow distribution. Various techniques, such as injecting radiographic contrast material and Evans blue solution, were also used to validate the results. 5. **Discussion**: The observed distribution of blood flow in the lung is divided into three zones based on the heights of pulmonary arterial, alveolar, and venous pressures. Zone 1 has no blood flow due to alveolar pressure, Zone 2 shows a linear increase in flow with distance, and Zone 3 also shows a linear increase but at a potentially slower rate. The pressure outside the distensible resistive vessels in Zone 3 is primarily alveolar pressure. Overall, the study provides a comprehensive understanding of how pressure changes affect blood flow distribution in the lung, which has implications for both physiological and pathological conditions.