Physiology: The Science of the Body

by Ernest G. Martin

We have now traced the food into the small intestine and will remember that up to this point the actual act of digestion has not gone very far. The enzyme of saliva has decomposed part of the starch, or perhaps all of it, into a complex sugar; the enzyme of gastric juice has split some of the very complex proteins into simpler ones. Of the final products of digestion none has yet been formed; their formation is the work of the small intestine. Into the upper end of it, as we have already seen, pancreatic juice and bile are poured from the pancreas and liver respectively. The pancreatic juice contains three enzymes; one of them is identical with that of saliva and completes the decomposition of any starch that the saliva has failed to act upon; the second is an enzyme which acts upon proteins, differing from pepsin of gastric juice in that it carries the splitting of proteins to completion; the third is the enzyme which acts upon fats. Fats are the most troublesome of all the foodstuffs from the digestive standpoint, because they are insoluble in water. Not only do they themselves offer difficulty, but they smear over the other food masses and make it hard for the digestive juices to get at them. Although the enzyme for digesting fat is secreted by the pancreas, its successful working depends on the presence of bile from the liver. If there is an obstruction of the bile passages, so that bile cannot be poured out into the small intestine, the digestion of fats stops and, along with it to a considerable extent, all the other digestive processes of the intestine. There is a familiar common condition known as jaundice, in which the bile passages become stopped, usually as the result of inflammation. When one is suffering from this condition, it is very important that not much fat be included in the diet; otherwise there is likely to be trouble with indigestion.

In addition to the pancreatic juice and bile there is a secretion known as intestinal juice which is poured out by small glands scattered along the wall of the small intestine. These secrete enzymes which convert the various kinds of sugar into the particular sugar, glucose, which is the one the body can use. The glands also secrete an enzyme which can carry on to completion protein digestion. Thus there is ample insurance that all the foodstuffs shall be made ready for the use of the body.

The next thing in order is for the digested foods to pass from the digestive cavity into the blood. This process is known as absorption. The first part of the alimentary tract from which absorption might occur is the stomach, but, as we have just seen, the process of digestion is not carried far enough in the stomach to fit the foods for going on into the blood stream. Absorption from the stomach is, therefore, undesirable rather than desirable. This is interesting because many condiments, particularly the hot spices and mustard, irritate the stomach lining in such a way as to promote absorption through it. Formerly this was looked upon as an argument in favor of the use of these condiments, but we now realize that it is rather an argument against them. The same applies to alcohol. Alcohol irritates the lining of the stomach and produces absorption through it. Unfortunately for those who desire a physiological argument in favor of the use of alcohol at meals this is a disadvantage rather than an advantage. The great region of absorption is the small intestine, where the digestive process is completed and where, therefore, absorption can properly go on. As to the method of absorption not a great deal needs to be said; there is a very delicate membrane lining the intestine, and just beneath this are the very numerous capillaries through which the blood is flowing; also in the spaces between the capillaries is the tissue fluid which drains into the lymphatic system. The products of protein digestion and glucose pass through the lining membrane of the intestine and through the capillary walls directly into the blood stream and are carried along in it. The digested fats, on the other hand, after passing through the intestinal lining enter the tissue fluids instead of the blood. Incidentally it should be noted that they are rebuilt into tiny fat droplets during the process of absorption, so that what we find in the tissue fluids of the intestine are true fats and not digestion products of fat. These tiny fat droplets pass away from the intestine by way of the lymphatic system. This, as we saw in the last chapter, drains finally through large lymphatic vessels into the great veins just at the entrance of the heart, and so the fat finally reaches the blood stream, but not so directly as do glucose and the digestion products of protein.

There is a reason for this difference in the handling of fats and the other foodstuffs which will be clear when we recall the point made in the last chapter that all the blood which flows through the walls of the intestine is collected in the portal vein and must pass again through the capillaries of the liver before entering the circulation at large. What happens is that all the digestion products, except the fats, pass through the capillaries of the liver, but the fat gets into the blood stream by another way and does not go through the liver en route; for some reason or other it seems to be to the advantage of the body that the fat should not be allowed to circulate through the liver capillaries. It is, as we shall see in a later chapter, a distinct advantage to have the glucose pass through the liver, and there is probably also a good reason for having the digestion products of protein take that course.

The digesting food is moved along the small intestine by contractions of its muscular wall, which travel along slowly in the form of a wave, very much as the wave of swallowing passes down the neck of a cow or horse. These waves do not occur regularly but only now and then; what happens is that several inches or two or three feet of the small intestine will be filled with a food mass which stays for a while at that place, digestion going on in it, and the digested food being absorbed through the wall. Both the digestion and the absorption are aided by a kind of churning motion made up of a series of contractions spaced about an inch apart along the part of the intestine where the food mass lies. These contractions form and disappear quite regularly at the rate of several a minute. They have the effect of squeezing the food mass rhythmically, but do not move it away from the place where it is lying. After this churning has been going on for a while it subsides, leaving that part of the intestine entirely quiet; then a contraction wave sets in at the end of the food mass toward the stomach and pushes the mass bodily along the intestine to a new section where the wave dies out and the churning motion begins again. This process is repeated with every food mass as it comes through from the stomach into the intestine until, finally, usually three or four hours after the first food has commenced to come into the intestine from the stomach, the whole mass has been propelled the length of the small intestine and what is left has been passed on through into the large intestine. The small intestine is about twenty-five feet long in man; flesh-eating animals of about the same size have intestines only about one-half as long; grazing animals the same size, on the other hand, have intestines two or three times longer than those of man; this intermediate length of the human intestine is ordinarily looked upon as indicating that man is adapted to a mixed diet, being neither strictly a flesh-eating nor strictly a vegetable-eating animal.

During the slow passage of the food mass through the small intestine the processes of digestion and absorption are completed, so that very little material enters the large intestine except the indigestible parts of the original food. These are made up mostly of cellulose and similar indigestible vegetable materials, but they include also indigestible fragments of gristle and other meat remnants. It is also worthy of note that the amount of water does not decrease much during the passage of material through the small intestine; in other words the material that enters the large intestine is about as liquid as that which passes from the stomach in to the small intestine. This does not mean that there is no absorption of water from the small intestine; in fact, water that we drink is probably absorbed quite rapidly; what it does mean is that enough water is secreted into the intestine from the blood in the lower parts to keep the whole mass liquid; this is of course important as an aid both to digestion and absorption.

The large intestine, as its name implies, is a tube of much larger diameter than the small; it begins low down on the right-hand side, passes up to about the level of the stomach, then across the body and down on the left-hand side, making thus a sort of inverted “U” in the abdomen; the space within the “U” is filled up by the loops and coils of the small intestine. The first part of the large intestine is devoted to the absorption of water; the result of this absorption is to leave the contents of the large intestine in a semidry condition; absorption of water goes on continuously, so that the longer the material remains in the large bowel, the drier will it become. In the ordinary course of events the material, as it is dried out, is passed along through the large intestine and at intervals, which should not exceed twenty-four hours, the accumulated material is discharged from the body.

Various facts about the functioning of the intestine which have seemed to many people mysterious are easily explained on the basis of what we have next to talk about, which is the presence both in the small and in the large intestine of flourishing colonies of bacteria. It has perhaps not occurred to most of us that we are in the position of involuntary hosts to enormous numbers of these microscopic plants, but that is exactly what we are. There is no possibility of preventing this, since the combination of warmth, moisture, and abundant food, which is afforded within the intestine, is the most favorable possible situation for many kinds of bacteria, and no effort on our part would enable us to be free from them. Fortunately, in the ordinary course of events, we are not affected one way or the other by their presence. In the small intestine their chief diet consists of the sugars which are produced as the result of starch digestion or are eaten directly. In connection with nourishing themselves from these sugars, they bring about what is known as alcoholic fermentation; that is, some of the sugar is converted into alcohol with a by-product of carbon dioxide. Exactly the same thing happens when yeast develops in a mass of dough; in the latter case the bubbles of carbon dioxide are what we are after, since they are what makes the bread light, the alcohol being driven off by the heat of cooking. In the alcoholic fermentation that goes on in our small intestines the carbon dioxide is a waste product and is passed on out of the body, but the alcohol is absorbed and used as fuel. Little has been said thus far about the physiological effects of alcohol. At this point all we care to say is that alcohol can be oxidized by the body tissues with the liberation of energy and so is a perfectly good fuel food. In the minute amounts in which it is absorbed from the small intestine it is utilized completely and there is not the slightest reason to suppose that it has any ill effects whatever. The objections to the use of alcohol, which have led to its abolition as a beverage in this country, depend on certain definite pernicious drug effects which it shows when consumed in any but the smallest amounts, and which destroy the usefulness of what would otherwise be a valuable article of human diet. Fermentation within the small intestine ordinarily goes on entirely unperceived by us. Certain foods, or the invasion of the intestinal tract by certain species of bacteria, may change the fermentation in such a way that irritating substances are produced which cause the movements of the small intestine to be very greatly increased; its contents are swept on through in very much less than the usual time and we have the condition known as diarrhea.

Within the large intestine bacterial action goes on fully as vigorously as in the small, and because of the smaller relative amounts of digestible food and of water a much larger proportion of the intestinal contents consists of the bodies of the bacteria and of the products of their metabolism. Since the sugars have been completely absorbed out by the time the food reaches the large intestine and the remaining materials upon which the bacteria can feed are more largely of the protein class, the bacterial action changes from fermentation to that which we commonly describe as putrefaction. This type of action, instead of giving rise to alcohol and carbon dioxide, produces a number of highly offensive compounds; many of these can pass through the lining membrane of the large intestine into the blood and so can circulate around the body. Of late years we have realized that these products of intestinal putrefaction are highly poisonous, especially if present in the blood stream in any considerable amount. The term “auto-intoxication,” which we run across frequently in health literature, means strictly the poisoning of the body by the products of intestinal putrefaction. Of course, the condition is much aggravated if putrefaction is allowed to go on for too long a time. The ill feelings which result from constipation and which in the life of very many people constitute a really serious impairment of health are the direct result of poisoning by these putrefaction products. The obvious remedy is the avoidance of intestinal sluggishness; unfortunately this is easier in theory than it often is in practice. Probably two conditions of modern life are chiefly responsible: the first is our tendency to make the diet more and more highly concentrated; that is, to leave out of it more and more the indigestible parts. The result is that not enough indigestible material enters the large intestine to make a sufficient bulk upon which the intestinal muscle can work in moving the mass forward. While bulk is accumulating, both absorption of water and putrefaction are going on, until by the time a sufficient mass is present, it is so dry that the muscles are not able to move it along and it has produced undue quantities of poisons. Obviously the way to treat this condition is by eating more indigestible material. For this purpose there is nothing better than apples in their season. The old proverb “An apple a day keeps the doctor away” has very sound common sense back of it. Apples are not ideal for this purpose for all people, since they frequently cause a distressing evolution of gas or even headache. Nearly everybody, though, by experimenting, can find a time of day at which an apple can be eaten without any digestive trouble whatever, and frequently with considerable benefit. Apples are valuable simply for their bulk of indigestible substance. The process of cooking converts most of this into digestible material, so that for this particular purpose they must be eaten raw. There are various other fruits such as figs, prunes, and raisins which function similarly and in addition have a direct stimulating effect upon the intestinal movements and so favor the discharge of material. Some people can eat popcorn to good advantage, although others suffer from gas distress if they do so.

The second condition of modern life which favors intestinal sluggishness is the sedentary habit which so many of us, both men and women, have. A vigorous outdoor existence is practically never complicated by auto-intoxication. Anyone who can maintain habits of active exercise will usually find himself troubled little by this condition. One further point should be made, and this can scarcely be over-emphasized, since it probably has as much to do as any other single factor in the avoidance of auto-intoxication; this is the development of regular habits in the matter of evacuation of the bowel. It is a general fact of the operation of smooth muscle that it readily develops certain habits. We have already seen an example of this in the behavior of the stomach in connection with mealtime. As we have noted, the stomach lies flabby between meals and enters into a state of tension just about the time that we are in the habit of eating. This adjustment is made no matter what particular habits individuals may have. Those who habitually eat only two meals a day will have this tightening of the stomach twice a day; others whose habit it is to eat five times a day will have a similar tightening five times in the twenty-four hours. Similarly the large intestine can establish a regular habit with regard to evacuation. This is best done in childhood, so that parents by insisting upon regularity in their children in this respect can usually assure them of a lifetime in which there will be little trouble from auto-intoxication. On the other hand, parents who are neglectful of their children’s welfare in this respect are laying up for them a lifetime of trouble and very much discomfort.