Figure 1 - Preepithelial defense.

In gastric and duodenal epithelia, hydrogen ions (H⁺) must cross the mucus–unstirred water layer–bicarbonate barrier before contact can be made with the surface of the epithelium. Diffusion of pepsin, but not H⁺, is blocked by mucus; however, H⁺ can be neutralized by bicarbonate ions (HCO₃^-) residing in the unstirred water layer. In contrast to gastric and duodenal epithelia, the preepithelial defense in the esophagus is poorly developed, having a limited mucus–HCO₃^- barrier to buffer back diffusing H⁺. [Source: Orlando RC. Esophageal epithelial defense against acid injury. J Clin Gastroenterol 1991;13(suppl 2):S1, with permission.]