Digestion, Absorption, Metabolism, Nutrition
( Originally Published 1954 )
Digestion is the process whereby food is changed in the alimentary tract into such simple states that it can be absorbed into the blood.
This process begins in the mouth. The mouth also has an important mechanical function in relation to eating. The teeth, with the aid of the muscles of mastication, grind the food into smaller and smaller particles; simultaneously, the salivary glands pour out their secretions which moisten and soften the dry foods.
The saliva is the first digestive juice to act chemically upon starchy foods. It is important to chew starches thoroughly so that as much saliva as possible may be mixed with the food; this initiates the chemical processes necessary for changing the food into an intermediate stage between starch and dextrose.
Most individuals, eating as they do—eating too quickly; eating fresh breads, soft buns, mashed potatoes, and washing these down with soups, gravies, and beverages—do not give the saliva a chance to act chemically on the food. Starch should always be eaten alone, without other foods in the mouth at the same time, and each morsel should be well-chewed and insalivated. Eating starchy foods in this manner would give the salivary glands sufficient opportunity to furnish their digestive product in quantity enough to accomplish the chemical work which they must perform.
The next step in starch digestion takes place in the stomach. There the saliva-food mixture continues to undergo the salivary chemical process, provided the stomach is not too crowded with protein food at the same time.
The stomach is chiefly an organ secreting the chemicals for digesting protein. In its inner surface are lodged glands which produce their characteristic chemicals—hydrochloric acid, pepsin and rennin for digesting protein and lipase for digesting fat.
The outer walls of the stomach are composed of muscular and connective tissues. The stomach also has a very rich blood and nerve supply. The blood supplies the glands with the constituent elements necessary for producing the digestive juices. The nerves furnish energy to the cells of the stomach for their mechanical and chemical functioning.
The normal, healthy stomach has a good supply of nerve energy. When the nerve energy is below par, a vicious cycle of digestive and motor disturbances may start. Abnormally delayed motion, or too much or too little secretion of digestive chemicals, is caused when the nerves of the stomach are hyperirritated or depressed.
The mechanical work of the stomach, like the mechanical work of the mouth, assures that the proper changes take place in the food to reduce it to simpler compounds. The stomach is more complex in its operation than the mouth. It retains food for a longer period of time before it completes its share of the process and sends the food on to the intestines.
The small intestine is the most important branch of the digestive system. In this part of the digestive tract, starches, proteins, fats, etc. are acted upon by very powerful secretions.
Its inner surface is so constructed that most of the digested food absorbs into the blood-stream by way of the intestinal wall. It is a tubular structure twenty-five to thirty feet long (sometimes longer), consisting, as does the stomach, of exterior muscular and connective tissue layers, and of interior mucous membranes, glands and villi (microscopic finger-like rootlets) through which absorption of digested food into the blood takes place.
The digestive juices of the small intestine consist of a number of highly active substances contributed by the liver, the pancreas and the inner surface of the intestine itself. This composite biochemical fluid completes the work of digestion which was left unfinished by the stomach and the mouth. In the small intestine, the major part of digestion and most of the absorption takes place. The pancreatic chemicals poured into the small intestine consist of four different kinds: trypsin, which digests proteins; lipase, which digests fat; amylase, which digests starches and intermediate dextrines; and chymosin, which digests milk.
The bile is produced by the cells of the liver. It is poured into the intestine for the purpose of aiding the digestion of fats. Bile also contains certain substances from the liver which help to stimulate the pancreas to produce its own digestive secretions. Bile also has antiseptic properties which act on germs that may have been swallowed with the food.
The digestive juice of the small intestine itself is the most powerful of all the digestive secretions. It contains several chemicals, such as lactase, which digests milk sugar; sucrase, which digests sucrose, cane and beet sugar; and a certain special chemical stimulator, enterokinase, which activates (makes chemically potent) the substance trypsin, for protein digestion.
The next important process in the vital chemical changes which food undergoes is its absorption into the blood-stream after its digestion is completed.
The inner surface of the small intestine is lined with minute finger-like projections called villi (there are about five millions of them). Just as the roots of a plant absorb nourishment from the soil, so the villi absorb the digested foods into the blood for the nutrition of all parts of the body.
The proteins are, at this stage of absorption, converted into chemical pounds called amino acids. As described in the chapter on protein, the amino acids are quite complex in their nature. They are of different chemical structures and functions, but they are soluble liquids which can be absorbed through the delicate villi into the blood.
The carbohydrates, starches and compound sugars are converted into glucose or dextrose when they are ready to be absorbed as a part of the blood. The fats are broken down into glycerine and fatty acids, in which forms they pass through specialized villi into the lymph.
The circulatory system, made up of blood vessels and lymphatics, carries the absorbed food materials into the important stations that come next. The liver receives all the nutriments from the blood except the fats, which enter directly from the intestines into the lymph stream. There the fats are organized into the type of fats characteristic of their component products, and are ready for assimilation into the body's economy—for storage, for energy, and for conversion into any other substances vitally needed or possible. As the absorbed nutriments enter the circulation, the liver plays a great role in further chemicalizing the protein and carbohydrate products.
The colon or large intestine plays the next important role in the alimentation of foods. The residues which cannot be absorbed through the villi move on into the colon, where a certain amount of absorption of moisture takes place, leaving a variable solid or semi-solid fecal mass for excretion.