Proposed mechanism for feedback regulation of ASL volume. (a) Evaporation reduces ASL volume, reduces the efficiency of mucociliary clearance, and increases ASL osmolality. (b) High osmotic permeability of the apical membrane allows water to be drawn into the ASL to restore osmolality and causes the cells to shrink. Shrinkage of surface columnar cells triggers release of vasodilator substances such as nitric oxide (NO), which increase fluid delivery by subepithelial microvasculature.