An important and previously unappreciated longitudinal convective-transport mechanism has been shown to operate in tube-bifurcation models geometrically and dynamically similar to upper and lower regions of the bronchial tree. The mechanism depends on the differences in shape that exist between inspiratory and expiratory velocity profiles in laminar and turbulent flow within the bronchial tubes and provides new and deeper physical insight into the flow of gases, aerosol particles, and heat in the airways. By use of dimensional analysis to relate the flows in the physical models to those in the real bronchial tree, the mechanism is shown to be important in normal breathing of gases and aerosols and is likely to be important also in high-frequency ventilation.