Scott Pryor, age 11, of Grand Ledge, Michigan, for his question:
Why doesn't the stomach digest itself?
The amazing digestive system is an orderly series of chemical processes. Step one begins in the stomach. This stretchable pouch secretes a corrosive acid strong enough to break up the molecules of meaty proteins. It also can dissolve zinc. These facts were known a century ago and ever since then people have been wondering why the stomach is not digested by its own powerful juices.
The stomach secretes the digestive juice pepsinogen and hydrochloric acid, one of the most corrosive acids. Obviously it has a system of defenses to protect itself from its own juices, otherwise it would be digested along with the food. Its defense system is two fold. The major protection is a layer of special cells that line every pinpoint of the stomach walls. The second defense is a fast rebuilding system that completely renews this protective lining every three days.
Under a powerful microscope, the stomach wall appears crinkled with folds and dented with pits. Neat little bumps, crowded cheek to cheek, cover the entire surface. These bumps are epithelial cells that form the defensive barrier. Like all cells, they have thin membranes that allow certain chemicals to filter in and out. Epithelial membranes are thought to be thin layers of fatty substance that prevent the strong gastric juices from getting through to digest the stomach walls.
These juices are manufactured in glandular cells deeper in the wail tissue. They ooze through tiny tubes that form those tiny pits on the surface. Both the tubes and their pitted openings are coated with protective epithelial cells. The moist . stomach lining, including the glandual and epithelial cells, is called the mucosa.
The surface membranes of the mucosa do not allow molecules of hydrochloric acid and pepsinogen to pass through. However, this barrier may be broken, possibly by the action of hydrogen ions in certain. chemicals. Experiments suggest that epithelial membranes may be destroyed by detergent type substances. As a rule, the barrier is repaired right away with new cells. But if the damage is severe, an ulcer forms and the corrosive juices really do start digesting the stomach.
Researchers are investigating other substances that may damage the stomach's lining. Two such chemicals may be digestive juices used by the small intestine. Normally the digestive process is a one way street. But sometimes the mixture from below erupts into the stomach, bringing bile salts that may damage the protective epithelial membranes. If this happens often, the mucosa may be unable to keep up with the repair work. Then its unprotected cells are at the mercy of the stomach's digestive juices.