Comparison of capsular and intra-alveolar fluid pressures in the lung

The fluid pressures measured in chronically implanted capsules were compared with fluid absorptive pressures in small degassed lung segments. Capsules were implanted, and bronchiolar catheters were placed at different vertical heights in the lungs of spontaneously breathing dogs. Increases in pulmonary capillary filtration pressure were produced by volume infusions of Tyrode's solution and increased left atrial pressure. A vertical gradient in fluid pressures was consistently observed with more negative pressures near the top of the lungs. Capsular fluid pressures averaged -7.82 cmH2O with a gradient of -0.60 cmH2O/cm distance up the lung. The intra-alveolar absorptive pressures averaged -14.4 cmH2O with a gradient of -0.78 cmH2O/cm distance up the lung. The fluid pressures in both the capsules and alveolar segments responded to changes in capillary filtration pressure (capillary hydrostatic pressure minus plasma colloid osmotic pressure). The overall change in these extravascular fluid pressures amounted to approximately 25% of the change in filtration pressure, although considerable variability in individual measurements was obtained. Because they respond to imbalances in Starling capillary forces, both the capsular and intra-alveolar fluid pressures may be considered a function of perimicrovascular tissue pressure in the lung.