SUMMARY
The processes which the food undergoes in digestion—conversion into condition to be absorbed by the body; in absorption through the walls of the intestines and stomach, and the metabolic processes which it undergoes in being converted into heat and energy and again broken down and eliminated as waste, are, in brief, as follows:
The Saliva begins the digestion of starches and sugars in the mouth, and continues this digestion for a time in the stomach.
The Stomach, when in normal condition, digests the proteins. If any proteins fail of digestion in the stomach the process is completed in the intestines. It has little absorptive power.
The Small Intestine digests and absorbs the fats and continues the digestion of starches, sugars, fats, and proteins, when this digestion is not completed in the stomach.
The large part of the food is absorbed through the small intestine, though a small part is absorbed through the walls of the stomach and through the large intestine.
Fats are almost entirely absorbed in the small intestine. They are absorbed through the lacteals and are carried into the blood-stream.
The intestines, aside from their work of digestion and absorption, excrete bile pigment, bile salts, mucus, and other decomposition products, also such food materials as are not digested.
The Liver. The proteins, the starches (converted into maltose), and sugars pass into the liver. The sugar (including the sugar in vegetables, milk, fruits and that used for sweetening as well as the carbohydrates which have been changed into maltose) is converted into glycogen in the liver, stored for a time, and again broken down into a condition in which it may be absorbed into the blood.
The proteins pass through the liver but are not acted on by this organ until they again return to the liver through the blood stream, after they have been partly oxidized in the tissues. The liver further oxidizes them, putting them into condition to be excreted by the kidneys and intestines.