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Under normal circumstances, the airway surface liquid (ASL) provides a low-viscosity environment for beating cilia to effectively clear mucus and pathogens from the airways. The composition, volume and osmolality of the ASL is therefore important for normal airway function.1-6
The respiratory tract is lined with ciliated cells interspersed with mucus-secreting goblet cells and other cells.1,2
A thin layer of aqueous fluid is formed between airway epithelial cells and the mucus layer. This is called the airway surface liquid (ASL).1,2
Optimal depth of the ASL allows cilia to fully extend and beat, thereby facilitating effective mucociliary transport.3,4
ASL height and the level of mucus hydration is mediated by a balance between sodium (Na+) absorption and chloride (Cl–) secretion across the apical membrane of airway epithelial cells.3-5
Normally, Cl– is secreted into the ASL via CFTR channels while Na+ is absorbed into epithelial cells by the epithelial sodium channel (ENaC). Both channels work in tandem to maintain optimal ionic balance.3-6
The flow of water accompanies the direction of Na+ transport via ENaC. For example, if the ASL is too ‘deep’ (overly hydrated), Na+ absorption increases ‘driving’ water into epithelial cells. When the optimal height of the ASL is achieved
(≈7 μm)3, Na+ absorption is reduced and Cl– secretion is increased via CFTR.3-6
Thus, coordinated regulation of Na+ absorption by ENaC and Cl– secretion by CFTR maintains the correct depth and composition of the ASL.