Estimated functional diameter of alveolar septal microvessels in zone 1

To estimate the functional diameter of alveolar septal microvessels in zone 1, we perfused isolated rat lungs with fluorescent latex particles of specific diameters (0.24, 0.49, 1.05, or 4.0 mu m) at pulmonary artery pressures (P-pulmart) that were either 5 or 10 cmH(2)O less than the air inflation pressure (P-pulmart 25 cmH(2)O). We then prepared samples for histology. Using a confocal, laser-scanning fluorescence microscope, we measured latex particle densities within the septal plane that ranged from 0.08 +/- 0.04 particles/mu m(2) (0.24-mu m diameter particles) to 0.02 +/- 0.01 particles/mu m(2) (1.05-mu m diameter particles). We found that 4.0-mu m diameter particles were not able to enter septa at all. Latex particles were not present in all alveoli. When P-pulmart was 5 cmH(2)O less than P-inflat, 32 +/- 6% of septa contained 0.24-mu m diameter particles, but, when P-pulmart was 10 cmH(2)O less than P-inflat, 5 +/- 6% of septa contained these particles. Percentages were smaller for larger particles. We conclude that, when P-pulmart is both 5 and 10 cmH(2)O less than P-inflat, the functional diameter of accessible septal microvessels is >1.95 but <4.0 pm. Furthermore, the number of accessible septa decreases as the difference between P-pulmart and P-inflat widens.