We examined the influence of asymmetry on the interaction of convection and gas-phase diffusion within the acinus of the lung. Single breaths of O2 were simulated by solving a differential equation for gas transport in two trumpet shaped units which were joined at a branch point and whose relative lengths and volumes were made to vary. Despite synchronous bulk flow to the from the units, in proportion to their relative volumes, the shorter unit always reached a higher O2 concentration (FO2) at end inspiration. Interdependence of gas transport at the branch point resulted in a falling FO2 within the shorter unit during expiration. The FO2 at the exit of the model therefore decreased progressively throughout expiration, simulating a sloping alveolar plateau. The simulations suggest that despite the relatively short distances separating parallel intra-acinar pathways, convective-diffusive interactions in the presence of asymmetry may produce substantial inhomogeneity in alveolar gas concentrations. Furthermore, the slope of the N2 plateau in the normal mammalian lung is explicable on the basis of the asymmetrical airway anatomy and well defined physical processes.