Gas mixing in a model of the pulmonary acinus with asymmetrical alveolar ducts.

An asymmetrical model of the human pulmonary acinus is described, in which elements of volume are represented by nodes joined by conductors permitting convective flow and molecular diffusion. The method of analysis permits simultaneous convection, diffusion, and dimensional change in any direction and requires only simple boundary conditions. Inspiration of O2 into a resident gas of 79% N2 followed by expiration was simulated at two flows. On expiration the slope of the alveolar plateau was 1.7%, and the alveolar N2 mixing efficiency was 97.0%. A symmetrical but otherwise similar model gave a slope of zero and a mixing efficiency of 99.9%. The patterns of gas concentration within the asymmetrical acinus during the respiratory cycle confirm and extend previous observations on the interactions between simultaneous convection and diffusion in asymmetrical structures (16, 21, 22). Even though these in combination within alveolar duct asymmetry can account for the slope of the alveolar plateau, they are insufficient to account for the failure of complete gas mixing found in normal subjects.