Fig. 1.

Schematic structure of an acinar airway with eight alveoli distributed along the annular ring (eight-alveolar model). Each airway (duct or sac) can be considered geometrically as a cylindrical object consisting of an alveolar sleeve with alveoli opening toward the internal cylindrical air passage. The diagram defines inner (r) and outer (R) airway radii (as in Fig. 1 in Refs. 7 and 37) and the depth of the alveolar sleeves (h). The ratio L = 0.765R is chosen so that alveolar size is the same along the airway and along the circumference (38). The shaded area shows lung air volume per alveolus V_a that comprises both the volume of alveolus proper and immediately associated duct volume.