The Physiology of Digestion

The Physiology of Digestion*

The digestion of food, which is an essen­tial experience in the daily life of each person, is something about which many theories and a multitude of strange notions have clustered.

By ROBERT A. HARE, M.D., Medical Director, ashington (D.C.) Sanitarium and Hospital 

The digestion of food, which is an essen­tial experience in the daily life of each person, is something about which many theories and a multitude of strange notions have clustered. Digestion is not a mysterious process. It is brought about largely by a series of chemical changes, modified by nerv­ous, physical, and physiological states.

Digestion takes place in a tubelike membran­ous and muscular tract, beginning with the mouth. Along this tract special physical struc­tures associated with varied secretions change the food progressively into its simple compo­nent parts. The mouth provides for the me­chanical division and crushing of the food, as well as for mild chemical changes. Here starches are partly broken down. The esopha­gus is the passageway to the stomach.

In the stomach a mixing process proceeds, and at the same time secretions from the stom­ach wall appear which partly digest the pro­teins eaten. Then the food is passed slowly from the stomach into the duodenum, where it is mixed with secretions from the wall of the duodenum, the pancreas, and the liver. In the twelve to fifteen inches of this portion of the bowel, relatively more digestion and actual ab­sorption takes place than in any equal extent of the entire tract. Secretions are formed, and digestion and absorption continue throughout the course of the small bowel, which is de­creasingly active. From the large bowel very little food material, but much water, is ab­sorbed. The final function of the large bowel is to be a receptacle to receive waste material until an appropriate time for its rejection.

It is proper now to discuss the agents that serve in bringing about the digestion of food. First, we may think of water. It is essential in digestion. It serves as a means of softening the food and holding it in suspension, and enters into the chemical reactions that take place in digestion. Many of the chemical changes in food come from adding or sub­tracting molecules of water.

Then we may think of the physical move­ments such as mastication, or the chewing of food, which divides it so as to make the in­dividual particles available to chemical attack. Peristalsis, which is a wavelike movement along the course of the digestive canal, is first observed in the esophagus in the act of swal­lowing. The movement observed throughout the digestive tract, however, is initiated in the stomach. In addition to the rhythmical waves there are other constricting and relaxing move­ments that occur in local areas of the bowel or in extended areas, producing effects often proportionate to their extent. The chemical aspects of digestion occur as the several diges­tive juices are mixed with the food—the chem­ical constituents of the juices softening and partially digesting the food for the action of the last agents we shall mention, the enzymes.

Enzymes are chemical ferments made by liv­ing cells, very complex in their own chemical structure, and possessed of the capacity of changing other chemical compounds without actually entering into the change themselves. They are perhaps the most important agents in digestion. They act commonly with great ra­pidity, to the degree that some physiologists have observed that we may regard their action as being almost explosive in nature. For ex­ample, the partially digested starches and pro­teins which leave the stomach, where there is no absorption, are acted upon by the enzymes in the duodenum within a few seconds or min­utes, so that a large amount of these substances is quickly ready for absorption. A not unim­portant percentage of this absorption actually takes place in the duodenum. There is evi­dence to make us believe that some small amounts of food substances may be absorbed which are not completely changed, and that digestion in such a case is completed in the blood stream by several enzymes found there.

Influence of Emotions on Digestion

The flow of digestive juices is directly in­fluenced by nervous states: The activity of the salivary glands and of the stomach glands re­sponds immediately to states of pleasure and happiness, or disgust and grief. This has been shown experimentally to be true both in ani­mals and in man. The flow is free in the pres­ence of pleasurable anticipation, whereas un­happy mental states may largely arrest the flow of these potent fluids. Worry disturbs diges­tion in respect to both the flow of digestive juices and the motility of the tract. The move­ments of the digestive organs are controlled largely by two sets of nerves, which are inti­mately connected with the general nervous control of the body. If one group is most directly influenced, there may be a comparative cessation of peristalsis and a sluggishness in function. Stimulation of the other nervous components may lead to a marked overactivity and almost uncontrollable diarrhea.

Food which is well masticated, aside from offering a better approach to chemical diges­tion, is less irritating to the intestinal mem­branes. It is not infrequently that a person who experiences irregular activity of the intes­tines and overactive elimination is promptly relieved by chewing his food more thoroughly. Copious hot drinks relax the tone of the stom­ach musculature. Iced water and chilled or frozen foods retard muscular and glandular activity. The maximum cooling effect from water drinking comes with its evaporation from the skin surface, and for this purpose it is best to drink it most freely between meals.

The appearance of food, whether it is pleas­ing or disgusting to the eyesight, whether it is served in clean and attractive dishes, whether it is pleasant to the senses of smell and taste, whether it is hot, lukewarm, or cold—all these may influence reflexly the function of the organs, and the effectiveness of digestion.

Eating between meals produces a steady draft upon the digestive juices. In certain dis­eases it is necessary at times to recommend frequent feedings during the day, but these feedings are always given at regular intervals and are small in amount. The introduction of food into the stomach at irregular periods, without giving time to dispose of a previous meal, interferes with the normal emptying cycle. Wrong habits of eating and the use of stimulating or irritating foods may produce a degree of irritability in the mucous membrane lining the stomach. Then when the stomach is empty, a sensation resembling a gnawing or aching may be felt. More food is eaten, when actually it would be better not to eat, but to allow the stomach a period of rest.

Immediately following the ingestion of a meal there is a marked increase in the flow of blood in and about the digestive organs. So that this may take place naturally, it is desir­able to rest or relax for a short time following a meal. Frequently mental workers notice a tendency to drowsiness after eating, which doubtless is the result of a lessened blood flow in the brain tissues while digestion is active.

There is a weakness experienced by some which is due to an insufficient amount of sugar in the blood stream. One whose meals are at reasonable intervals does not experience this sensation. Those who attempt to work with insufficient food or at a time long after the last meal often experience a marked "gone­ness," or exhaustion. For example, those who work without breakfast may feel weak or may have a headache during the late forenoon. If the blood is examined at this time it is found to contain less sugar than is normal, which is the cause of the profound weakness and tired­ness. There are individuals whose capacity for utilizing sugar has been so altered that they need to eat more often than others. They do not have a normal sugar-storing capacity. Such need medical guidance.

Anger Disturbs Blood Stream

In anger, the distribution of the blood is changed. Sugar is made available to the mus­cles, and feats of strength are sometimes ac­complished in an angered state which would be impossible otherwise. The activity of the di­gestive organs is usually arrested in anger. It is best not to eat in such a state.

X-ray studies of the digestive tract made when one is afraid or worried or terrorized, compared with those made at a time when he is at ease and at peace with his surroundings, show a marked difference in form. In the first case the intestines are narrowed; in portions there are tightly constricted segments, and short sections may be entirely closed. In a state of relaxation, the size of the intestine is normal, gently curved, constrictions are seen at intervals, representing the natural action of the muscles in the walls, and spasm or exten­sive contraction is not observed.

Digestion is accomplished in the operation of natural laws. As one studies the functions of the human body with an open mind, he finds that the principles set forth by inspiration and those discovered by experimentation and observation agree. Our understanding of each is made more accurate as we study more deeply. "Natural science is a treasure house of knowledge from which every student in the school of Christ may draw."—"Christ's Object Lessons," p. 125.


Alvarez, Walter C., M.D., "Neivous Indigestion," William Heinemann, Ltd., London.

Bayliss, W. M., D.Sc., F.R.S., "The Nature of Enzyme Action," Longmans, Green & Co., New York.

Howell, William H., M.D., Sc.D., "A Textbook of Physiology," thirteenth ed., Sec. VII: "The Physiol­ogy of Digestion and Secretion," Saunders, Phila­delphia.

Ivy, A. C., M.D., American Journal of Digestive Diseases and Nutrition, Vol. I, p. 845: "The Applied Physiology of the Gastro-Intestinal Innervation."

Joslin, Elliott P., M.D., "The Treatment of Diabetes Mellitus," pp. 61-76: "Obesity and Diabetes," Lea & Febiger,• Philadelphia.

Sansum, William D., M.D., Hare, Robert A., M.D., and Bowden, Ruth, "The Normal Diet and Healthful Living," Chap. V: "How the Body Uses Food:" Chap. VI: "Why the Body Needs Water ;" Chap. VII: "The Elimination of Waste," Macmillan, New York.

Starling, Ernest H., M.D., F.R.S., "Principles of Human Physiology," Chap. XXVII : "Nutrition," Churchill, London.

Wright, Samson, M.D., F.R.C.P., "Applied Physi­ology," pp. 553-560: "Mechanics of the Alimentary Canal," Oxford University Press, London.

*A discussion, with parallel source references, on Section 5, "Physiology of Digestion," of the book "Counsels on Diet and Foods."

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By ROBERT A. HARE, M.D., Medical Director, ashington (D.C.) Sanitarium and Hospital 

February 1939

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