Regulation of Pulmonary Capillary Blood Volume by Pulmonary Arterial and Left Atrial Pressures

We used single-breath CO diffusing capacity to study the effect of changes in pulmonary arterial and left atrial pressures on pulmonary capillary blood volume in 15 supine dogs whose lungs were perfused with a nonpulsatile pump. Effect of pulmonary arterial pressure: When left atrial pressure measured relative to the bottom of the lungs ≤ alveolar pressure, increasing pulmonary arterial pressure increased diffusing capacity markedly (mean, 0.71 ml/min x mm Hg per mm Hg change in pulmonary arterial pressure). When left atrial pressure ≥ alveolar pressure plus the height of the lungs, increasing pulmonary arterial pressure had less effect on diffusing capacity, although blood flow was increased through a wide range (mean, 0.22 ml/min x mm Hg CO per mm Hg increase in pulmonary arterial pressure). Effect of left atrial pressure: At zero flow, increasing left atrial pressure increased diffusing capacity markedly; when pulmonary arterial pressure was high, increasing left atrial pressure had no significant effect on diffusing capacity. Increasing bronchial arterial pressure from 0 to 150 mm Hg or inhaling 10.5% CO 2 had no effect on diffusing capacity. Injection into the pulmonary artery of glass microspheres decreased diffusing capacity only when left atrial pressure was low; their effect was exaggerated when pulmonary arterial pressure was high in the control state.