Gastric mucus, bicarbonate and pH gradients in mucosal protection.

The evidence is reviewed that mucus has the capacity to delay diffusion of acid across it. It may provide an environment adjacent to the gastric mucosa in which the relatively small amounts of bicarbonate secreted by the epithelium are able to nullify acid diffusing towards the mucosa. This combination creates a near-neutral zone immediately adjacent to the epithelium. Evidence in favour of this hypothesis is supports the idea that mucus and bicarbonate may together afford a first-line defence against luminal acid and pepsin, but equally it seems likely that this "barrier" can provide only part of the complete picture of mucosal protection.