In the stomach the process of digestion is continued, a large part of the protein being broken down into a form suitable for absorption. The digestive action of the stomach is twofold mechanical and chemical. The circular and longitudinal muscles of the stomach contract and relax with a rhythm like motion. In this way the food mass is moved back and forth, from one end of the stomach to the other, so that it is all churned up and mixed thoroughly with gastric juice.
The digestive ferment òr enzyme of the gastric juice is pepsin. It acts only on proteins or albuminous foods such as meat, fish or eggs. It is secreted by small glands all over walls of the stomach. As it acts only in an acid medium, the stomach glands also secrete hydrochloric acid for this purpose. The character of the gastric juice secreted in the different parts of the stomach varies considerably. This is especially true of its acid content. In the middle section, the secretion is rich in acid, while in the cardiac region and the pyloric end or part of the stomach which opens into the small intestine, the cells which appear to secrete the hydrochloric acid are few in number if they are present at all. Hence the juice secreted in these regions must be neutral, or according to some authorities, even slightly alkaline.
The muscular movements likewise vary considerably in the different parts of the stomach. The wavelike movements which cause the food to be mixed thoroughly with gastric juice begin in the middle portions and travel towards the pyloric end to the pylorus. Thus the food is pushed forward towards the pylorus and then back again as the muscles of the stomach constrict. In this way the food in this part of the stomach is thoroughly mixed with the gastric juice.
On the other hand, the food in the cardiac end of the stomach where it first enters is not moved by peristalsis or wavelike movements. Therefore it does not become as quickly mixed with gastric juice as in the lower region. Moreover, since the gastric juice secreted in this portion contains but a small amount of acid, if any, the food remains in the same neutral or slightly alkaline state that it was in when swallowed. It remains here for some time a half hour to two or more hours so that the ptyalin of the saliva may continue its work on the starches. This is contrary to the former belief that all salivary digestion was stopped by the acidity of the gastric juice as soon as it reached the stomach.
In the meantime the pouchlike portion of the stomach or fundus is slowly contracting upon the food mass and by degrees is moving it toward the pyloric region. As the food is digested the pylorus opens and lets it pass into the small intestine, and in the process the pylorus tends to open more and more frequently until finally it may open to allow food to pass which has not been acted upon by the gastric juice. However, x-ray examinations have shown that the pylorus opens only in the presence of acid in the intestine. Therefore it is probable that normally the food is allowed to pass into the intestine only when the protein is saturated with hydrochloric acid.
Just how long food remains in the stomach, and whether it will be completely emptied before the next meal, depends on several factors.
Health in general and of the stomach in particular are important factors. The mental and physical state at the time of eating also influences digestion. You will recall that we have previously stressed the need for relaxation and pleasant surroundings in order to insure good digestion. Under normal conditions, however, much depends upon the kind of meal eaten. Generally speaking a small meal or snack may disappear from the stomach in one to four hours, but if you adhere to the customary three meals a day, in spite of our pleadings to the contrary, a food meal may remain in the stomach for six or seven hours.
Under ordinary circumstances, if each type of food is eaten separately, the protein remains in the stomach longer than the starches and sugars, fats longer than protein and a mixture of protein and fat longer than either alone. This is probably due to the fact that fat has a tendency to retard the movements of the musculature of the stomach and the secretion of the acid gastric juice. However, fats that are emulsified or broken down into tiny globules pass on to the intestine more quickly than those in large masses. This is also true of the softer and more liquid fats.
In addition to its role in digestion, the acidity of the gastric juice has another all important function. It partially disinfects the food mass. It has been proven that when, because of some abnormality, the hydrochloric acid of the gastric juice is decreased, bacteria may multiply rapidly in the stomach.
The acidity of the food mass or chyme as it is called, when it leaves the stomach, also influences the flow of pancreatic juice, which is one of the digestive juices which acts upon food in the small intestine.
To sum up the functions of the stomach: it acts as a storehouse for large amounts of food,a whole meal, for example and then passes it on partially digested, in small quantities to the small intestine. It serves as a place for the continuation of starch digestion by saliva which was begun in the mouth. It starts the digestion of proteins and possibly of fats by breaking them into smaller particles. And lastly it acts as a disinfectant through the germicidal properties of the hydrochloric acid of the gastric juice.