Pulmonary Gas Transplant Time
The experimental evidence presented by Wagner et al. (1) does not imply that there is an interaction between the bulk flow and diffusion. On the contrary, the C-shaped curve in Fig. 1 suggests that the mechanism of transport is by convection alone [a more detailed discussion of convective dispersion is given in (2)]. The fact that the shortest transit time is about half that obtained from a calculation based upon the average velocity would also result if the velocity profile in the ducts are nearly parabolic; when the Reynolds number of the flow is less than about 2100, the velocity profile is nearly parabolic and the maximum velocity within the duct is nearly twice the average velocity. Furthermore, because Taylor diffusion (3) involves only radial diffusion, the minimum transit time does not change; that is, the minimum transit time remains equal to the distance between the larynx and the alveolus, divided by the velocity of the fastest streamline.