Diffusion, convection and their coupling as gas transport mechanisms in the lungs: a model experiment

The roles of diffusion and convection in gas transport were studied by means of a lung model with a hollow bronchial cast of a dog. He, Ar and SF6 were chosen as indicators with different diffusivities. Axial dispersion of a heavier gas was much better, but the concentration gradient in a static gas phase was greater for a heavier gas, i.e., there was a larger stratified inhomogeneity for a less diffusible gas. Crossings of He and SF6 concentration curves were observed between diffusion distances of 6 and 8 cm, which could be assumed to be a critical point of conversion where either convective mixing or diffusion favored gas transport. The results observed also suggested that the effective diffusion coefficient and speed of respiration affected the efficiency of gas transport.