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The distribution of pulmonary blood flow is influenced by gravity, regional lung expansion, and hypoxic pulmonary vasoconstriction. However, these factors cannot completely explain the three-dimensional distribution of blood flow in the lung. The present study was designed to see whether anatomically related factors could contribute. Regional blood pressure vs. flow curves were determined in 100-230 small parenchymal samples (0.3-0.4 ml) from 12 isolated perfused dog lungs held at constant inflation pressure. In each region four blood flows were measured using radioactively labeled microspheres, and the four corresponding regional perfusion pressures were determined by correcting the measured perfusion pressure for hydrostatic effects. There were considerable differences in the slopes of the pressure vs. flow curves among lung regions. Dorso-caudal regions of the lung had higher vascular conductances than ventrocephalad regions, independent of the vertical orientation of the lung or the inflation volume during injections of microspheres. Thus the distributions of regional vascular conductances were related to the anatomic location and were not related to gravity, nor were they caused by nonuniformities in regional lung expansion or by hypoxic vasoconstriction or edema.

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Intrapulmonary distribution of ventilation/unit lung volume was studied in 28 volunteers in the sitting, supine, or right lateral decubitus position, either awake or anesthetized-paralyzed and mechanically ventilated. We found significant differences between the awake state and anesthesia-paralysis with mechanical ventilation in 1) intrapulmonary gas distribution, and 2) the vertical gradient of regional functional residual capacities for the subjects in the lateral decubitus position, but not for those in the sitting and supine positions. The effect of increasing the tidal volume on distribution of ventilation was significantly different 1) between the three body positions for a given state, and 2) between the two states for a given body position. The data suggest thoracoabdominal mechanics are different in the three body positions and that anesthesia-paralysis and mechanical ventilation may cause a different pattern of expansion of the respiratory system than spontaneous breathing in the awake state.