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 I). Methods: We perfused isolated rat lungs (P-A 25. P-PA 25. P-LA 0 cm H2O) with fluorescent latex particles of specific diameters (0.49, 1.05, 2.0, 4.0, or 10 mu m) and then prepared samples for histology. Using a confocal, laser-scanning fluorescence microscopy, 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: Particles densities in septa ranged from 0.06 +/- 0.02 particles per mu m(2) for 0.49-mu m-diameter particles to 0.007 +/- 0.004 particles per mu m(2) for 4.0-mu m-diameter particles. The 10-mu m particles did not enter septa. Calculations based on these data suggest a septal microvessel functional diameter of 6-8 mu m. Conclusions: In a previous study, conducted at the same value of P-inflat, but with P-PA set at 15 or 20 (5 or 10 cm H2O into zone I), we estimated the capillary diameter to be 1.7 mu m. Thus, the septal capillary diameter seems to increase by three- to fourfold as P-PA is raised to equal P-inflat.