Perfusion of alveolar septa in isolated rat lungs in zone 1

The combination of high inflation and low vascular pressures in zone 1 lungs is assumed to collapse alveolar vessels, making them inaccessible to vascular liquid. To test this assumption, we perfused isolated rat lungs in zone 1 (n = 5) with fluorescent albumin solution (inflation pressure = 25 cmH2O, pulmonary arterial pressure = 10 cmH2O, left atrial pressure = 0 cmH2O; flow = 0.11 +/- 0.06 ml.100 g body wt-1 x min-1) and rapidly froze them. Histologically, 33 +/- 19% (SD) of alveolar septa fluoresced, demonstrating that the perfusate had not been excluded. However, we could not resolve whether the fluorescence originated in the septal microvascular lumen or in the adjacent perimicrovascular interstitial space. To address this issue, we perfused an additional lung with horseradish peroxidase (HRP) and examined it by transmission electron microscopy. HRP filled interstitial spaces around septal vessels and extraseptal alveolar corner vessels, but because the septal vascular lumina were too compressed, we were unable to determine whether they also contained HRP. Therefore we perfused two additional lungs with particles of colloidal gold (0.05 microns diam). Using transmission electron microscopy, we found gold particles in 15-25% of septal vascular lumina, demonstrating that septal vessels were at least partially accessible in zone 1. Our interpretations is that filtration in zone 1 may occur from septal vessels and extraseptal alveolar vessels. Furthermore, results of the HRP study suggest that the perimicrovascular interstitial space is less compressible than the septal vascular lumen.