Quantitative ultrastructural study of the rabbit lung: exposure to 60% oxygen for 21 days.

When rabbits were exposed to 60% oxygen for 21 days, arterial PaO2 showed statistically insignificant changes at 6-8 days (69 +/- 5 Torr) and 14-16 days (62 +/- 5 Torr); at 21 days the PaO2 was significantly decreased (56 +/- 5.4 Torr) as compared to 79 +/- 2 Torr at 0 time of exposure. No significant alterations in PaCO2, pH, or hematocrit were present. Morphometric techniques for electron microscopy showed no qualitative or quantitative alterations in lung morphology except for a significant decrease in the volume per lung of the capillary endothelial lumen, possibly reflecting an altered recruitment of capillaries. The total capillary volume per lung however did not differ significantly between oxygen and control-treated rabbits. The physiologic finding of a decreased oxygenation after 60% oxygen exposure suggests that changes in oxygenation precede the morphologic parameters studied.