The increase in total cross-sectional area in the distal airways of the human lung enhances
the mixing of each tidal breath with end-expiratory gas volume by slowing bulk flow and
increasing gas diffusion. However, this transition also favors the deposition of airborne
particulates in this region because they diffuse 600 times slower than gases. Furthermore,
the persistent deposition of toxic airborne particulates stimulates a chronic inflammatory
immune cell infiltration and tissue repair and remodeling process that increases the …