Convective gas transport in the pulmonary acinus: comparing roles of convective and diffusive lengths.

To investigate the relative importance of convection and diffusion in the transport of oxygen in the pulmonary acinus, it is often useful to locate the transition from convection-dominated to diffusion-dominated transport. Traditionally, this is done by estimating the values of a Peclet number. This dimensionless number compares the bulk ductal flow velocity at an acinar generation with a diffusion velocity over a characteristic length scale. Here, we revisit the convection-diffusion transition by comparing the relative importance of convective and diffusive lengths. We introduce the ratio of such lengths (L(conv)/L(diff)) to quantify the extent of convective transport in the acinus over an inhalation phase. We distinguish between convection along the acinar airways and within alveoli, respectively. Results for L(conv)/L(diff) suggest that convection in acinar ducts may play a potential role in more peripheral airways compared with values obtained for a Peclet number. Within alveoli, however, independent of acinar depth, oxygen transport is governed by diffusion as soon as molecules enter within alveolar cavities.