Figure 1.

Transepithelial Na⁺ transport drives alveolar fluid clearance. Na⁺ enters the cell interior passively following an electrochemical gradient via Na⁺ channels, which are located at the apical membrane of alveolar epithelial cells. The Na⁺ ions are then actively pumped out of the cells by the Na⁺/K⁺-ATPase in exchange for K⁺ ions, which leave the cell afterwards via basolaterally localized potassium channels. Thus, there is a net movement of Na⁺ ions from the apical (air-faced) to the basolateral (blood/interstitium-faced) side of the alveolar epithelium. This creates osmotic forces, and, consequently, water follows out of the airspaces across the epithelium either paracellularly via tight junctions or transcellularly via aquaporins. The figure has been modified from [2].