Human airways are lined with a film of liquid from 5–100 μm in depth, consisting of a
periciliary sol around and a mucous gel above the cilia. Microscopical studies have shown
the sol to be invariably the same depth as the length of the cilia, and we discuss possible
reasons for this. The composition and sources of the airway surface liquid are also
described. In addition the forces regulating its volume are analyzed. Several airway
diseases are characterised by dramatic changes in the volume and composition of airway …