Functional diameters of alveolar microvessels at high lung volume in zone II

To estimate the functional diameter of alveolar microvessels, we perfused isolated rat lungs with fluorescent latex particles (1 diameter/lung) at inflation, pulmonary arterial, and left atrial pressures of 25, 30, and 0 cmH2O, respectively. We used confocal microscopy to count latex particles within septal microvessels and flow cytometry to count particle concentrations in venous outflow. We found 1-, 2-, and 4-micron-diameter particles within septal vessels of 45 +/- 12, 31 +/- 12, and 25 +/- 9%, respectively, of examined alveoli. Particles of 5-micron diameter were absent from septal vessels but were present within a small percentage of corner vessels. Particle concentrations in the venous outflow for 1-, 2-, 4-, and 5-micron-diameter particles were 54 +/- 28, 67 +/- 32, 2.2 +/- 0.3, and 0.4 +/- 0.3%, respectively, of the arterial inflow. Particles with diameters of 6 or 10 micron were absent from venous outflow. Our results suggest that, under these conditions, the functional diameter of the septal microvessels is approximately 4 micron and that the diameter of the adjacent corner vessels is slightly larger but <6 micron.