A reappraisal of boundary conditions assumed in pulmonary gas transport models

Solutions of the classic pulmonary gas transport equation are presented in which a true 'no-flux' boundary condition is specified throughout the respiratory cycle. For the particular models studied it is demonstrated that diffusive mixing is incomplete at end expiration, and that such stratified inhomogeneities give rise to a realistic alveolar plateau for a simulated N2 washout test. The reasons for the disparity of the present findings with those obtained by contemporary workers are explained by critically examining the boundary conditions conventionally assumed at the alveolar wall.