Estimated functional diameter of alveolar septal microvessels at the zone I-II border.

Objective: To measure the functional diameter of alveolar septal microvessels under conditions in which the pulmonary arterial pressure and the lung inflation pressure are equal, at 25 cm H2O (zone I-II border), and to compare these results with those obtained when inflation pressure exceeded arterial pressure by 5 or 10 cm H2O (zone l).
Methods: We perfused isolated rat lungs (PA 25, PPA 25, PLA 0 cm H2O) with fluorescent latex particles of specific diameters (0.49, 1.05, 2.0, 4.0, or 10 microns) and then prepared samples for histology. Using a confocal, laser-scanning fluorescence microscope, we measured latex particle densities within the septal plane of individual alveoli. We compared these particle densities with those in arterioles supplying the septa and calculated the density ratio. We fit curves produced by the Verniory equation to these ratios to estimate the septal microvessel functional diameter.
Results: Particle densities in septa ranged from 0.06 +/- 0.02 particles per microns2 for 0.49-micron-diameter particles to 0.007 +/- 0.004 particles per microns2 for 4.0-microns-diameter particles. The 10-microns particles did not enter septa. Calculations based on these data suggest a septal microvessel functional diameter of 6-8 microns.
Conclusions: In a previous study, conducted at the same value of Pinflat, but with PPA set at 15 or 20 (5 or 10 cm H2O into zone I). We estimated the capillary diameter to be 1.7 microns. Thus, the septal capillary diameter seems to increase by three- to fourfold as PPA is raised to equal Pinflat.