Morphometry of capillaries in three zones of rabbit lungs fixed by vascular perfusion

In large lungs in the upright position, three functional conditions originated by gravity-related changes in the arterial (Pa) and venous (Pv) pressures with respect to alveolar air pressure (PA) are defined as follows: Zone 1 (apex), where PA > Pa > Pv, Zone 2 (middle), where Pa > PA > Pv, and Zone 3 (bottom), where Pa > Pv > PA. This article attempts to clarify the anatomic correlates of these "zonal" changes.

Rabbit lungs were placed in one of these zones by adjusting the inflation, pulmonary arterial and left atrial pressures and fixed by vascular perfusion of osmium to preserve the original configuration of alveolar surfaces and blood vessels. The specimens were analyzed by electron and scanning microscopy and by morphometry of the capillary lumina and septal thickness.

We noticed the existence of large isolated corner vessels and of corner pleats of the alveolar wall in Zones 1 and 2, but not in Zone 3. These pleats and corner vessels contained bundles of dilated capillaries. By re-construction of serial sections we found that these always represent functional, not anatomically-fixed shortcuts between arteries and veins which permit a bypass of parts of the alveolar capillary network. In Zones 1 and 2, which showed similar morphology, septal capillaries outside of the corners were partly closed and when open, frequently had slit-like configuration. In Zone 3, all septal capillaries were wide open, of circular outline and of similar dimensions. Morphometry showed great dissimilarities of capillary size in Zones 1 and 2, and homogeneity in Zone 3, as proven by analysis of the size distribution curve of capillary sizes.

There are substantial reversible morphological changes in the configuration and dimensions of capillaries in the three zones, involving differences in capillary diameter, number of capillaries open and homogeneity of capillary size. Of particular interest are the corner vessels and septal corner pleats present in Zones 1 and 2, which represent multiple, small functional arteriovenous shortcuts whose function is to maintain the patency of the right-to-left vascular path during high alveolar air pressures. It is noted that human patients placed in a respirator have their whole lungs in Zone 2. The pleat has a potential for working as an efficacious gas exchanger.