Figure 1.

Ion transport across alveolar epithelial cells. Na⁺ and Cl⁻ ions enter the apical membranes (lumen) of alveolar epithelial cells (both Type I and Type II) via Epithelial sodium channels (ENaC) and Cl⁻ channels such as cystic fibrosis transmembrane conductance regulator (CFTR) located in the apical face of the cell (interstitium). The Na⁺ ions are driven by an electrochemical gradient created by the energy-dependent Na/K-ATPase. K⁺ ions, brought in by the Na/K-ATPase pump in exchange for the Na⁺ ions, leave the cell passively through K⁺ channels located in the basolateral membrane. CFTR: Cystic fibrosis transport regulator; ENaC: Epithelial sodium channel.