The term digestion includes all the chemical changes that occur while food is being prepared for use by the body. Another process called absorption embraces the transfer of digested food to the blood stream. Absorption of the products of digestion does not take place in the mouth or stomach, as only preliminary changes take place in these organs. It is commonly believed that a very small amount of water or alcohol may be absorbed through the stomach wall, but some workers claim that water and saline solutions are not absorbed through these tissues. The several types of food are acted upon at the same time by the digestive fluids beyond the stomach which have been described and are brought to final digestion and absorption in the small intestine. The process begins in the duodenum and is believed to be practically complete when the food mass reaches the ileum or lower portion of the small intestine.
How food is actually carried through the membranous wall of the digestive organs is not understood in detail. Some of it passes the wall of the bowel by physical and chemical processes; some is believed to pass between the cells; probably, some is taken up directly by the cells of the lining mucous membrane.
When digestion is complete proteins no longer exist as such, because they have been split into amino acids. These important foods are taken up by the tiny blood vessels in the wall of the intestine, and carried to the liver. There some of them are changed promptly so as to add to the sugar supply of the body, while others circulate in the blood so as to be available to the tissues, to be used both in the growth of new cells and in replacing those damaged in the activities of living.
Starches and sugars are finally absorbed as simple sugarschiefly glucose. These simple sugars are carried in the blood stream for a short time following a meal, and the level of the blood sugar may be raised slightly until the surplus sugar is changed to body starch. This is accomplished largely by the liver, and the starch known as glycogen is stored in the liver and muscles for use as need arises.
Digestion has brought about very interesting changes in the fats eaten. They have been changed to glycerine, soaps, and fatty acids dissolved in bile. These substances enter the cells lining the villi or minute finger-like processes covering the surface of the intestinal wall, and are rebuilt into neutral fats suitable for the use of the human body. The white corpuscles of the blood pick up these particles of fat and transport them into the lymph channels which ultimately empty the fatty solution into the blood stream through the thoracic duct, which unites with the veins near the heart. When the thoracic duct is tied off the absorption of fat is not stopped completely, so it is probable that some is absorbed directly into the blood stream. A few hours after absorption the excess fats have been removed from the circulating blood and are added to the deposits of fat in the body, in the form of human fat.
A food eaten does not retain its identity when digested. Eating the tissues of an animal, for example, does not guarantee that the human tissues similar to that ingested will be built up. Absorption does not change that which has been digested, but merely delivers it to the blood, the “master tissue” of the body. Then as need exists the several products are built into body cells, or used in maintaining the activity of cells already functioning. If more has been absorbed than can be used, some of it may be lost through body excretions, or more often it is changed and stored as body fat.
The absorption of minerals is probably brought about by physical and chemical transference of salts or compounds through the intestinal wall. How the vitamins enter the body is not clear. It may be that they are attached to some portion of the food in which they are found. After these necessary foods have been removed from the contents of the bowel, and the available water has been withdrawn, a useless residue remains. How this is expelled from the body will be the subject matter of a later discussion.