On equation of gas transport in the lung.

Based upon Weibel's lung model, an equation which describes one-dimensional gas transport in the lung is derived. It is shown in this equation that the gas transport in the airways is due simultaneously to bulk motion and longitudinal diffusion. The longitudinal diffusion consists of the usual molecular diffusion and an apparent diffusion which arises from nonuniform distribution of the gas to the airways and a profile-interaction effect of the gas due to Taylor's mechanism in each individual airway. Expressions for the apparent diffusion coefficient are obtained. This apparent diffusion, which has been neglected in almost all previous studies of gas transport in the lung, is shown to be of dominant importance in the upper airways.