Model analysis of intra-acinar gas exchange

A previously described multibranch-point model, incorporating branching asymmetry within an acinus, has been extended to include gas exchange at the alveolar surface. Using a transport equation for simultaneous convection and diffusion within the gas phase and independent perfusion of all nodes, we obtained steady-state solutions for the temporal and spatial distributions of O2 and CO2 tensions within an acinus during a respiratory cycle. Results for conditions corresponding to both rest and moderate exercise indicated a significant inhomogeneity of gas concentrations within a single acinus. The coefficient of variation of PACO2 at end-inspiration during exercise reached 11.3%. Despite this non-uniformity the computation of a negligible PAO2 - PAO2 difference indicated no impairment in gas exchange. The simulations are consistent with the hypothesis that in the normal lung the whole acinus acts functionally as a gas exchanging unit and ventilation-perfusion inequality has an interacinar basis.