The stomach is a reservoir and food processor. In it, most of the chewed and swallowed food turns into a mash that is slowly plunged into the digestive tract.
Food enters the stomach through the muscular one-way ”valve”, the esophageal sphincter. As food enters the stomach, strong muscles of gastric wall squeeze the food, while acids and enzymes, produced by the stomach mucosa, disintegrate it. All these activities last as long as the food is in the stomach; the muscular activity of the empty stomach is small, and there is practically no release of acid or enzyme.
The “processed” substance slowly exits from the stomach through another muscular “valve”, a pilic sphincter, and enters the duodenum. The duodenum is a tube of about 250 mm long, and extra enzymes are excreted in it that enhance the digestion process before the “mash” passes into the small intestine.
Usually, it takes about five hours for the food to exits the stomach and the duodenum and enter the small intestine. An excessive production of strong acids and enzymes that act in the stomach and the duodenum may damage the gastric mucosa. In addition, in some people, this mucusa can easily be irritated by certain types of food, medication or fluids. In most cases, this irritatin causes only well-known symptoms of poor digestion. However, in some of the more serious cases, the chemical compounds in the stomach can cause the development of an ulcer on the stomach or duodenal mucosa.