Analyses of sequential filling and emptying of the lung.

We simulated sequential filling and emptying of a lung lobe (Fukuchi et al., 1980) by solving a differential equation for simultaneous convection and diffusion within a solid expansile model of an asymmetrical acinus. For a given degree of asymmetry subtended at a fixed branch point the shorter units are consistently better ventilated (by diffusion) despite homogeneous lung expansion. Thus a helium bolus inhaled at 80% of vital capacity (VC) results in a higher helium concentration (FHe) at full inflation within the shorter units. A bolus inhaled at 20% VC results in relatively greater FHe in longer units. On expiration diffusive interaction between parallel pathways at the branch point results in a range of alveolar plateaus whose slope varies inversely with the volume at which the helium bolus is inhaled. This interdependence of gas transport among parallel pathways contributes to the "first in-last out' pattern observed experimentally. Comparison of radially expansile with axial-radially expansile models does not produce qualitatively significant differences. However, equilibration of gas concentrations during breathholding in the model occurs more rapidly than under experimental conditions indicating that other mechanisms may also contribute to the observed pattern of lung filling and emptying.