Pulmonary microcirculation: differences in endothelia of subpleural and alveolar capillaries.

Portions of the endothelia in the alveolar and subpleural capillary networks appose either the adjacent alveolar epithelium (thin sides) or the supportive interstitial spaces (thick sides) of the alveolar septa or the visceral pleura, respectively. In both the alveolar and subpleural capillaries, endothelial vesicle numerical densities and the amount of endothelial cell attenuation were greater on the thin sides than on the thick sides. Morphometric comparisons between the subpleural and alveolar capillaries on their respective thin sides defined greater endothelial attenuation (34% of endothelial surface compared to 14%) in the subpleural capillaries. Further, endothelial vesicle numerical densities were less in the subpleural capillaries (118/micron3 compared to 226/micron3). Vesicle diameters, on the other hand, were greater in the subpleural capillary endothelia (0.82 micron compared to 0.64 micron). The differences between subpleural and alveolar capillary endothelia on their respective thick sides were analogous to those described for the thin sides of capillaries. Endothelial attenuation and vesicle diameters on the thick sides of the subpleural capillaries exceeded those on the thick sides of the alveolar capillaries, and subpleural capillary endothelial vesicle densities were less than those of the alveolar capillary endothelia. The marked differences between the alveolar and subpleural capillary endothelia extend previous reports of differences in densities between the capillary networks and serve to further establish regional variations in the pulmonary microcirculation.