Biologist William K. Purves of Harvey Mudd College responds:
Parietal cells in the mucosa, the inner cell layer of our digestive tract, secrete hydrochloric acid (HCl) into the stomach's lumen, or cavity. The solution in the lumen may have a pH of one or less¿10 times as acidic as pure lemon juice. This fact raises two distinct questions: how can the mucosa form HCl without being attacked in the process? And why doesn't the acid, once present in the lumen, attack the mucosa?
First, the mucosa doesn't get digested as it secretes the HCl because the pH within the parietal cells remains near neutrality by the following mechanism (note that all parts of the mechanism take place simultaneously): (1) Potassium ions, K+, diffuse passively from the parietal cell into the lumen. (2) An active transport pump brings K+ back into the parietal cell, simultaneously secreting H+ from the cell to the lumen. As much K+ returns by this route as leaks in (1). (3) Chloride ions, Cl-, diffuse passively from the cell to the lumen, and their negative charges balance the positive charges of the secreted H+.(4) An exchanger on the opposite face of the parietal cell balances this loss of Cl- by importing Cl - from the blood in exchange for bicarbonate ions (HCO3-). (5) Within the cell, water reacts with carbon dioxide (CO2) to form carbonic acid (H2CO3), which dissociates into H+ and HCO3-. The figure (right) summarizes these steps. In summary, the secreted HCl arises from H+ (from water) and Cl- from the blood. Both accumulate only in the lumen and not in the parietal cell.
Second, HCl in the lumen doesn¿t digest the mucosa because goblet cells in the mucosa secrete large quantities of protective mucus that line the mucosal surface. Basic electrolytes, such as HCO3-, trapped inside the layer of mucus neutralize any HCl that penetrates the mucus.
This mucus layer also protects the mucosa from digestive enzymes in the lumen, but why don¿t the enzymes do damage before they are secreted from the exocrine cells of the mucosa? Because they are never present within the cells! The cells synthesize and secrete proenzymes that are larger than the active enzymes. Once in the lumen of the stomach, these proenzymes become active enzymes only by having part of the proenzyme molecule digested away, leaving the "business" part of the molecule to function. For example, the digestive enzyme pepsin arises when HCl cleaves the proenzyme pepsinogen.
In summary, HCl in the stomach lumen accomplishes four things. It helps break down ingested tissues for attack by digestive enzymes; it provides the correct pH for the action of those enzymes; it converts a catalytically inactive proenzyme to an active enzyme (as we just saw); and it destroys invading microorganisms in the stomach contents.
As the stomach contents pass from the stomach to the small intestine, their acidity is rapidly neutralized by the addition of HCO3- produced by the pancreas¿a good thing, too, because the mucosa of the small intestine has no other protection against HCl.
Despite their sophistication, the stomach¿s protections are not infallible. Heliobacter pylori is a bacterium that thrives in the highly acidic environment of the stomach. An H. pylori infection can start a lesion that HCl and digestive enzymes can exacerbate, resulting in an ulcer.
Answer originally published November 13, 2001.