Figure 2 - Epithelial defense.

Some of the recognized epithelial defenses against acid injury are illustrated. Structural barriers to hydrogen ion (H⁺) diffusion include the apical cell membrane and intercellular junctional complex. Functional components include intracellular buffering by negatively charged proteins and bicarbonate ion (HCO₃-) and H⁺ extrusion processes (Na⁺/H⁺ exchange and Na⁺-dependent Cl^-/HCO₃^- exchange) for regulation of intracellular pH. Na⁺ = sodium ions. (Source: Modified from Orlando RC, Dobrucali AM. In: Feldman M, Orlando RC, eds. Atlas of Esophageal Diseases, 2nd ed. Philadelphia: Current Medicine, 2002: 99, with permission.)