Figure 1: Schematic representation of the airway epithelia. The major cell types in the proximal airways are represented along with their most important ion transport systems. The airway surface fluid (ASF) is the fluid in which cilia moves to enable mucociliary clearance. The correct depth of ASF is maintaned by regulated absorption and secretion of ions and water. The epithelial sodium channel (ENaC) and the Na+/K+ ATPase membrane proteins are involved in Na+ absortion. The TMEM16A (also known as anoctamin 1) and the cystic fibrosis transmembrane conductance regulator (CFTR) on the apical membrane and the sodium-potassium-chloride cotransporter (NKCC) on the basolateral membrane are involved in Ca2+- and cAMP-mediated Cl- secretion, respectively. The inward Na+ gradient established by plasma membrane Na+/K+-ATPase pumps also propels the countertransport of H+ by sodium/proton exchanger (NHE) which along with bicarbonate cotransporters extrude the excess of acid accumulated by cellular metabolism and by various H+ (acid equivalents) leak pathways conducts to intracellular pH maintenance.