Surfaces and volumes of alveolar tissue under zone II and zone III conditions

The functional anatomy of alveolar septa has been studied in rabbit lungs fixed by vascular perfusion under middle zone II and zone III conditions at a constant lung inflation level of 60% of total lung capacity. Capillary volume increases down the vertical axis of the lungs in both conditions and is larger by about 30% in zone III than in zone II lungs. The concomitant changes of free alveolar, epithelial, and capillary surface areas are small. Capillary volume increases within the observed range of pressures neither by recruitment of collapsed capillaries nor by unfolding of pleats of alveolar septa and capillary walls, but rather by deformation and distension of tissue: scanning electron micrographs reveal a more conspicuous bulging of capillaries in zone III than in zone II lungs. Accordingly, the mechanical structure of alveolar septa appears to be largely consistent with a sheet-flow model.