Convection, diffusion and their interaction in the bronchial tree

Abstract

The bronchial tree is an asymmetric multi-generation conduit for air to pass through. Breathing is a time-varying, cyclic process. Therefore, gas transport in the bronchial tree during breathing is a problem with both spatial and temporal complexities. However, it is possible to gain a fair understanding of this complex process by examining the roles of bulk convection, molecular diffusion and the various modes of their interaction. Convection brings the inspired gas into the peripheral region of the lung and removes the mixed inspired-resident gas from the lung. During the post-inspiratory and post-expiratory pauses and in the very distal regions of the lung, molecular diffusion alone is responsible for the mixing process. Convection and diffusion together are responsible for establishing a quasi-steady front of the inspired gas in the peripheral lung region. The position of the front depends on lung volume, tidal volume, inspiratory flow-rate as well as diffusivity of the inspired gas. Taylor-type dispersion, while certainly existing in the lung, and despite its gas mixing potentials, is of questionable effectiveness in the overall gas transport process in the bronchial tree. The various modes of interaction between convection and diffusion are likely to improve gas mixing in the bronchial tree and have an effect on the phase II of the single-breath washout curve.