|MadSci Network: Anatomy|
Hi Rob, good question --
The stomach is made of many layers, starting with a thin, tough layer on the outside called the serosa. It then has three layers of involuntary muscle which mechanically grind and mix swallowed food. The innermost lining is called the epithelium which consists of many specialized cells. One type, the parietal cell secretes hydrochloric acid (HCl). This cell type and others secrete enzymes, including pepsin, which degrade proteins. However the epithelial layer also contains many mucus-secreting cells. The mucus creates a protective layer over the stomach's epithelial lining. This layer buffers the underlying stomach tissues from the stomach's acidic environment.
As to why the enzymes don't degrade the stomach wall - Pepsin, and other stomach enzymes only have activity at low pH, namely when they enter the stomach's lumen. The pH near the epithelial surface is much higher as it has the overlying layer of mucus to buffer it from the acid. Thus, the enzymes do not become active until they have safely gotten beyond living tissues (or so one hopes..).
As you could imagine, damaging the protective mucus layer would allow the stomach's acid and enzymes to come in contact with the underlying tissue. If this happens often enough, an ulcer can develop. The acid erodes through the lining, creating a painful condition that can become life threatening should the ulcer penetrate all the way through the stomach (a perforation), or should it eat away at a major blood vessel in the wall of the stomach (lots of bleeding). We used to think that ulcers were caused by "psychological" factors such as stress and "mental imbalances." However, we now know that a microbe called Helicobacter pylori causes many ulcers. The organism lives in the stomach's mucus layer, and "does it's thing" by causing long-term irritation of the epithelial lining.
-L. Bry, MadSci Admin
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