Why Does The Stomach Not Digest Itself,And If It Did Digest Itself Completely What Would Happen?

Question

I Would Like To Know.

Answer 1

How Does the Stomach Avoid Digesting Itself?

The stomach is made of protein, yet it houses the most powerful protein digesting enzymes known. How can it manufacture and contain these enzymes without serious damage to its own tissue?

A microscopic examination of the stomach wall shows that its epithelial layer is not smooth. It is folded into thousands of tiny pits, called gastric pits, that penetrate into the surrounding muscle layers. On the surface, it looks rather like a well used pin-cushion, but the internal structure of these pits is quite complex.

Virtually all the epithelial cells facing the lumen (the inside of the stomach), and all those lining the upper parts of the pits, produce large quantities of mucus. Mucus is a mixture of water and a protein that causes it to become viscous and slippery. Any membrane that produces mucus is called a mucous membrane or a mucosa, and so the epithelium of the stomach is called the gastric mucosa. The gastric mucosa produces so much mucus that it insulates itself from the contents of the stomach. This is the simple answer to the question, "Why doesn't the stomach digest itself?", but a little examination will show that it is not complete.

When the food enters the stomach it is usually close to neutral pH, and it lacks protein-digesting enzymes. These have to be added to the food. Clearly, the acid and the enzymes must be made in the tissues of the stomach, since there are no other glands feeding in. There are thus still two unresolved problems: how can cells manufacture and store these highly destructive chemicals without destroying themselves? and how can they deliver them to the lumen of the stomach if there is a protective mucus layer in between?

A closer look shows that there are three distinctive types of cells in the mucosa at the bottoms of the gastric pits. There are still many mucus secreting cells, but there are two other types known as chief cells and parietal cells. Chief cells are responsible for producing pepsin, but they protect themselves by producing it in an inactive form called pepsinogen. Pepsinogen does not break down proteins unless it is converted to pepsin in a chemical reaction. Chief cells, however, do not contain the necessary reactants to stimulate the conversion. Thus they can safely store up the enzyme until it is needed in the lumen.

Parietal cells are specialized to produce concentrated hydrochloric acid (HCl), which they store in protective vesicles. It is possible to produce phospholipids that are resistant to strong acids. Lysosomes, for example, contain such acids.

When food enters the stomach, hormones stimulate the secretion of both pepsinogen and HCl. The HCl immediately begins to convert the pepsinogen to pespin. At the same time, hormones and nerves stimulate the muscles of the stomach wall to contract rhythmically. As the muscles contract, they squeeze the gastric pits, creating increased pressure. The gastric pits act as thousands of tiny water pistols, squirting their contents into the lumen before much pepsin has been created, and certainly before it has time to do serious damage. As the muscles relax again, the pressure in the gastric pit becomes lower. Mucus produced by the cells in the neck of the pit is drawn down and forms a plug. Thus, pepsin and acid from the lumen cannot re-enter the gastric pit.

Meanwhile, in the lumen, more and more of the pepsinogen is converted to pepsin. This is a positive feedback reaction, since pepsin itself actually catalyzes (accelerates) the conversion. Within a matter of seconds, all the pepsin is in active form, and beginning to work on the ingested proteins.

While the mucous layer lining the lumen is fairly effective at protecting the stomach wall, it is not impervious. Cells of the mucosa are constantly being damaged, and have an average lifespan of only a few days. The basal layer beneath the epithelium must bud off enough new cells to replace the entire gastric mucosa every week. If the rate of destruction is too high, perhaps because of insufficient mucus, or the rate of replacement too low, the acid and pepsin may begin to destroy the underlying tissues of the stomach wall, causing an ulcer.

Answer 2

excellent answer nalaredneb! highly accurate, informative, and very clear.

Answer 3

It does to some extent, but it keeps replacing its lining.