Evidence is presented that the design of the air passages in respect of dimensions and the ratios of diameters of parent to daughter branches and angles of branching closely approximate the theoretical optimum for mass movement of gas down to the level of terminal bronchioles (0.6 mm in diameter). Distal to this point, the geometry of the passages support the hypothesis that gas movement is by molecular diffusion.
The single-breath nitrogen washout experiment using oxygen indicates an inhomogeneity of gas mixing. Studies of the pattern of inhomogeneity using neon, sulphahexafluoride, and an aerosol of di-2-ethylhexyl sebacate as an indicator gas—along with varying periods of breath holding—indicate that about two thirds of the inhomogeneity is due to stratification or imperfect diffusion. The remaining third is due to regional inhomogeneity consequent upon variations in the transit times of mass movement of gas.
A detailed analysis of the multiple-breath nitrogen washout procedure points to stratified inhomogeneity as the major cause of departure from uniformity of distribution of ventilation.
Evidence points to stratified or diffusion inhomogeneity as the major ventilatory defect in chronic bronchitis, especially when centrilobular emphysema is present.