The digestive system modifies the foods which are eaten so that they may be absorbed and distributed throughout the body for heat and energy and for growth and repair. This modification of the foods is essentially a chemical one, but the various mechanical actions assist in the process. The mechanical movements of the canal move the food along to be acted upon by the series of digestive juices. They also mechanically break up food particles and mix the food with the digestive juices, and finally expel the waste residues from the body. These mechanical movements are all accomplished by the central muscular layer within these hollow structures. It is a series of contractions and relaxations of these small muscles which force the food along its course, as shown in Fig. 40. This is called peristalsis.
Along the entire digestive system there are digestive juices, called enzymes, which carry out the chemical modification of the foods. Digestion starts in the mouth where mastication (chewing) breaks up the food and where the saliva is secreted. Saliva dissolves certain foods, protects the lining of the mouth, and provides lubrication, but it also contains an important enzyme called ptyalin. This is an enzyme important for the digestion of starches. Even though the food remains in the mouth but a short time, the action of ptyalin continues within the stomach in the saliva-saturated food.
The next in the series of digestive juices are those of the stomach. Here are produced pepsin and hydrochloric acid. These substances are made in the small glands in the mucosal layers of the stomach wall. Pepsin is necessary for the digestion of proteins. Although the hydrochloric acid is not actually an enzyme, it is a most important digestive juice. It aids the peptic action in the digestion of proteins.
Bile is secreted into the intestinal canal from the liver by a small tube which drains into the upper portion of the small intestine. Bile is basically waste products from metabolic action in other parts of the body, but in the liver it is altered to become one of the most important chemical digestants. The bile salts are essential for the digestion of fats.
The pancreas gland secretes a digestive juice which contains three specific enzymes: amylop- sin digests sugars, trypsin digests proteins, and steapsin aids in the digestion of fats.
In the small intestine the enzyme erepsin further aids the digestion of proteins, and some sugar digestion is further accomplished. As in the stomach, the mucosa lining of the small intestine contains numerous small glands which secrete this digestive juice.
In the colon, digestion still continues. This organ contains abundant quantities of living germs. These are continually present from soon after birth and are essential to life. They live on undigested food and liberate fermentation enzymes which continue the digestive process.
Throughout the entire tract there is some absorption of the digested foods. However, the upper portions of the tract are concerned mainly with secretions of digestive juices, and most of the absorption of digested foods takes place in the terminal portions of the tract, mostly from the ileum. The intestinal contents are liquid until they reach the large bowel, where they become more solid as the water is absorbed.