Interaction of convection and diffusion in pulmonary gas transport

Normal human subjects inspired various volumes of a normoxic argon mixture containing low concentrations of several biologically inert tracer gases with markedly different diffusivities (helium, neon, and sulfur hexafluoride). The behavior of Ne, Ar, and SF6 could be predicted on the basis of axial dispersion due to differences in diffusivity. For example, neon, having the highest diffusivity of the three, was more uniformly distributed within the bronchial tree than either argon or SF6. The behavior of helium, however, was not consistent with predictions based solely on axial diffusion. Contrary to expectation, the early portion of expiration was helium enriched while gas assumed to come from the alveolar regions contained relatively less helium than the other gases. Results of this study suggest that radial diffusion during convective bulk flow may play a significant role in intrapulmonary gas transport if relative diffusivity is extremely large. We conclude that diffusion gradients do exist within the bronchial tree during normal quiet breathing and that these gradients become less significant as inspired volume increases.