Alveolar vessel behavior in the zone 2 lung inferred from indicator-dilution data

To gain insight into the changes occurring in alveolar vessels when alveolar pressure exceeds venous pressure at the downstream end of the alveolar vessels (zone 2), we compared the uptake of serotonin and the extravascular volume accessible to ³HOH (Qev) under zone 2 and 3 conditions in isolated dog lung lobes. We also examined the influence of occluding some of the small pulmonary arteries with 58- to 548-μm-diam beads on the serotonin uptake and Qev. We found that, with the bead embolization, both the serotonin uptake and the Qev were reduced, whereas the change from zone 3 to 2 reduced serotonin uptake but did not change Qev. A plausible explanation for these observations is that the beads occluded vessels that were relatively large compared with those in which significant transvascular ³HOH exchange and serotonin uptake take place. Perfusion ceased in the collection of capillaries normally served by the obstructed arteries. Thus the extravascular water and the serotonin uptake sites downstream from the obstructions were not accessible to the indicators during the short time interval of the indicator passage through the lung. On the other hand, the change from zone 3 to zone 2 resulted in the collapse of small individual capillary segments within the alveolar vessel bed. Since the serotonin does not readily diffuse from the vessels through the tissue, it could not reach the endothelial cells of the collapsed capillaries. However, since the distances for diffusion between collapsed capillaries and neighboring perfused capillaries were small, the more highly diffusible ³HOH had access to the same Qev under both zone 2 and 3 conditions. According to this interpretation, at least part of the increase in vascular resistance that occurs when the alveolar-venous pressure difference is increased in zone 2 is the result of the collapse of some alveolar vessels as opposed to only a narrowing at the capillary venous junction.