Physiology: The Science of the Body

by Ernest G. Martin

CHAPTER XV
THE SERVICE OF SUPPLY OF FOOD

IN order for the blood that circulates through the body to pass on to the body cells the materials which they need, it is evident that the blood itself must have some source from which to obtain the materials. Our present task is to examine this source. In Chapter IV we talked about food and its uses; here we are concerned with the way in which the foodstuffs are taken into the body and prepared for use. We know that the material which we take in is part solid and part liquid, giving rise to the familiar distinction between food and drink. For our present purposes this distinction has no importance and will not be made. What we do have to note is that of the complex mixture of materials which makes up any ordinary meal, some of the substances are ready to be taken up at once by the blood and to be distributed around the body; others have to go through a preliminary course of preparation. All of the dietary accessories, so far as we know, pass from the digestive organs into the blood without change. All or nearly all of the energy-yielding foods, on the other hand, must have a preliminary preparation to which we give the name of digestion; the operation of digestion being to break down the complex foodstuffs that are eaten into simpler materials of which the body can make use. Digestion is made up of a series of chemical changes by which the large molecules of the original foodstuffs are broken into smaller and simpler molecules. This breaking down of large molecules into smaller is a very common kind of chemical process. A feature of it is that under ordinary circumstances the breaking down of the large molecule goes on very slowly, but if the right conditions are provided the breaking down proceeds rapidly. The particular condition which is necessary is the presence in the solution where the large molecules are breaking down of something which will hurry up the process. There are a good many kinds of substances which have this ability to hasten molecular decomposition; those that do it in carrying on digestion are given the name of enzymes. We do not know just how the enzymes act; we merely know that when they are put along with the substance which is to be decomposed, it goes to pieces very much more rapidly than if none of them was present. The enzymes themselves are not used up in the process, so if time enough is allowed, a very small amount of enzyme can bring about the decomposition of a very large amount of material. In our bodies the whole digestive process consists of a succession of decompositions of complex materials into simpler ones under the speeding-up influence of enzymes. We have a number of different kinds of foodstuffs to be decomposed and a corresponding number of enzymes.

The process begins in the mouth; here the food is chewed and moistened with saliva before being swallowed; both the chewing and the moistening are important to insure good digestion later on. The enzymes have no particular ability to penetrate a mass of food material; what they do is to attack it from the outside and work in as it decomposes. Since enzyme action is thus a surface action, it is evident that the larger the surface the more efficient will be the action. Chewing is nothing in the world but a mechanical breaking up of the food to get the largest possible surface. We have sufficient proof of its importance in the digestive disturbances that arise as the direct consequence of improper chewing. Of recent years it has been realized that undernutrition, particularly in children, is often a result of the failure to chew the food properly; it has been found, furthermore, that bad teeth or improperly shaped mouths are very frequently responsible. For this reason in most of our large cities dental clinics are being established for the purpose of inspecting and, if necessary, caring for the mouths of school children. The result of this work is to improve the general average of health among children simply by increasing the extent to which the food is chewed. This fact, together with that described in Chapter XIII as to pus pockets, should impress upon both parents and teachers the fundamental importance of proper care of the teeth. This includes not only the prevention of decay by daily thorough cleaning and the securing at intervals of not more than six months of dental inspection with treatment where necessary, but also in the case of children with deformed jaws the special treatment necessary to bring the teeth into position for effective chewing. Too much stress can scarcely be laid on the importance of these precautions.

The moistening of the food with saliva is likewise important to good digestion. As a matter of fact, without it even the act of swallowing would be impossible. One can easily prove that insufficient moistening prevents swallowing by eating a rather large quantity of a very dry food, like crackers. A common habit of people is to moisten the food with water which they drink instead of waiting for it to be properly moistened by saliva. Contrary to popular belief there is absolutely no objection to drinking water at mealtime. In fact, the presence of the additional liquid in the alimentary tract is probably beneficial rather than the reverse; there is, however, a very serious objection to using water for washing down half-chewed food. The best possible way to judge whether the food is sufficiently chewed is by observing whether it has been sufficiently moistened so that it will be swallowed easily. If so, the chances are that the chewing has been sufficient. Between mouthfuls as much water may be taken as one desires, although if ice water is drunk, it should be held in the mouth until the worst chill is taken off before being swallowed, so that it will not chill the stomach. Since the comfort of drinking ice water is in the cooling of the mouth and throat and not at all in the cooling of the stomach, this increases rather than diminishes the enjoyment one gets from a glass of it on a hot day. So much satisfaction can be obtained by proper drinking habits, that it is a pity to allow the health to be injured by improper habits to such an extent as to necessitate, as frequently happens, the complete abandonment of water drinking with meals.

The saliva is primarily for the purpose of moistening the food, but besides this it is also a definite digestive juice, because it contains one of the digestive enzymes. This enzyme, to which is given the name ptyalin, acts upon starch, changing it to sugar, but not to the particular sugar of which the body makes use. Perhaps we should digress for a moment to say that there are chemically a number of sugars. These are in many respects alike, although they also vary a good deal among themselves. For example, some are much sweeter than others; glucose, which is a sugar that can be made from cornstalks and other plant products, is not nearly so sweet as cane or beet sugar, although one is fully equal to the other so far as nutritive value is concerned. Of all the various kinds of sugar that exist, only those in the glucose class can actually be used by the body, so that all other sugars have to be changed by digestion into a glucoselike sugar before passing on into the blood. The enzyme of saliva converts starch not into glucose but into a more complex sugar to which is given the name of maltose. It, therefore, starts digestion, but does not carry it through to completion. Starch digestion begins during the course of chewing and mixing the food with saliva; it goes on while the food is being swallowed, and for a short time after it enters the stomach, being stopped there sooner or later by means which will be described in a moment. No other real digestion occurs in the mouth, although the chewing and moistening are of very great importance in preparing the food for the digestion that is to come further along.

The food that is swallowed passes down the esophagus and enters the stomach. Before speaking of this it will be necessary to recall what was said in Chapter VII about the behavior of the stomach walls. Between meals these are in a relaxed and flabby state, with the opposite walls lying more or less in contact. There is usually a little liquid and some swallowed air in the stomach, so that it is not actually empty, even when we speak of it as being so. Just before mealtime the walls of the stomach draw up so that instead of a flabby bag we have a fairly tense tubular organ. It is at this time that the contractions of the stomach wall, which we recognize as hunger, begin to come on. There is a sphincter muscle between the esophagus and stomach which closes the opening tightly and so prevents the pressure within the stomach from forcing gas or liquid back up into the esophagus. This sphincter opens automatically only in connection with the act of swallowing. Every time we make a swallowing movement a sort of wave passes down the esophagus, and when this wave arrives at the stomach the sphincter relaxes, allowing whatever was moving down the esophagus to enter. If one watches a horse or cow in the act of swallowing, the rather deliberate progress of this wave down the neck can be followed. What happens as we eat a meal is that every mouthful, as it is swallowed, is passed through the sphincter into the stomach and room is made for it by a gradual relaxation of the stomach wall, so that, as we saw in Chapter VII, the pressure of the stomach against its contents stays fairly steady, in spite of the fact that more and more material is being taken into the stomach from the esophagus. One result of this behavior of the stomach is that the first food that is swallowed is nearest to the stomach walls, and that which is swallowed afterward is nearer the middle, being inclosed on all sides by the previously swallowed food. This is fairly important because the gastric juice is secreted by glands in the wall of the stomach and so will get in contact first with the food that is swallowed first, and only afterward will reach the food that was swallowed later. Occasionally the sphincter between the stomach and the esophagus relaxes unexpectedly; this is said to happen more often in smokers than in nonsmokers. The result is that some of the sharply acid stomach contents are forced up into the esophagus and vigorous swallowing is necessary to crowd them back down into the stomach. The burning sensation which accompanies this is known as heartburn, although, as we have just seen, it is really entirely a matter of the esophagus and has nothing whatever to do with the heart.

Although the stomach carries on a certain part of the work of digestion, its primary purpose, as we shall see presently, is to serve as a storage place into which a considerable amount of food can be placed in a few moments and so enable us to do our eating at three or four definite meals instead of little by little throughout the day. When we rise from table after any meal, we have in our stomachs roughly one-third of our total food supply for the day, the exact proportion depending, of course, on our individual habits as regards our distribution of food-taking among the three meals. During the two to four hours following the meal this accumulated material in the stomach will be passed along little by little to the small intestine, which is the real digestive organ of the body. In the course of this time there will be some additional digestion within the stomach, but not enough to prepare any foodstuffs for actual use by the tissues. The outlet from the stomach to the small intestine is guarded by