Pulmonary defense mechanisms preventing chronic bacterial infection. The lung is depicted as an inverted funnel, reflecting the relative surface area of distal versus proximal airways. The mechanical-clearance-of-mucus hypothesis is shown on the left. The schema depicts discrete mucus and periciliary liquid layers and ascribes to the epithelium a volume-absorbing function. The chemical shield hypothesis is shown on the right, with the epithelium depicted as having a salt- but not a volume-absorbing function to produce the hypotonic ASL required for defensin activity.