Diagrammatic representation of aquaporin expression in select respiratory tissues. (a) Distal lung showing alveolar airspace, type I pneumocytes containing AQP5 in the apical membrane, type II pneumocytes with no known aquaporins, and underlying capillary containing low levels of AQP1. (b) Submucosal secretory gland acinus containing apical AQP5 and basolateral AQP3 or AQP4. This figure is modified and reproduced with permission (9).