Physiology: The Science of the Body

by Ernest G. Martin

full growth because it has been denied the amino acids necessary for making new protein retains the power of growth, so that even though it may be long past the ordinary age of maturity it can go on growing as soon as the necessary materials are provided. The dependence of growth on certain dietary accessories was spoken of in Chapter IV and need not be repeated here.

The final use of food is as a source of energy for carrying on metabolism. A good deal was said about that in an earlier chapter, but there are a few additional points to be brought out here. The energy for metabolism can come from any of the foodstuffs; these are present in the blood stream in the form of sugar or fat or amino acids. A moment ago we saw that in the case of the amino acids the nitrogen is removed before they are ready for use as fuel. After this has happened the part that remains is so similar to sugar that it can be thought of as the same material and will be so considered by us. We have then to follow only the two food substances, sugar and fat, through their use by the body cells. Fat will be dealt with first since we have less to say about it. It passes from the digestive tract into the blood stream in the form known as an emulsion; all this means is that it is broken up into very tiny particles which are kept from running together by some sort of a film. In the case of the fats in the blood it is likely that this film is composed of ordinary soap. In this state the fat circulates in the blood stream until it is taken out by the tissues and burned. In the process of this burning some very poisonous substances are likely to be produced, but this happens only when large amounts of fat are being burned by themselves. If sugar is present and is being burned at the same time, there is no danger. Ordinarily in the body sugar is always present, but in a certain disease, diabetes, about which more will be said later, the body is not able to burn sugar as well as it ordinarily does, and under these circumstances poisoning from the products of the fat burning is apt to happen. This makes up, in fact, the serious danger in diabetes. As we all know, fat is chiefly important as the form in which food is stored in the body against a time of future need. We shall return to the way in which the body does this after we have spoken of the use of sugar as fuel.

Sugar is the great fuel substance of the body. About two-thirds or our food ordinarily consists of starchy materials which are digested into sugar. When we add to this the sugarlike remains of all the protein food which is not actually used for the repair of tissues we see that this substance makes up the great bulk of the material which is carried by the blood to the tissues. This material is handled in the body in an interesting way which depends on the curious fact that although sugar is the most important fuel for living cells they cannot endure its presence in them or in the fluids surrounding them except in very small amounts. Sugar, as we all know, is very soluble in water, and it would be perfectly possible for the blood to dissolve all the sugar that enters it from the digestive tract and simply carry it in solution until the tissues withdrew it for their needs; but this would mean that immediately after every meal the percentage of sugar in the blood would mount up to a high point from which it would gradually sink as the sugar was taken out, to mount again after the next meal when absorption began again. This does not actually happen because it is prevented by the liver, which has as the most important of its many functions that of storing the sugar that is taken up by the blood from the digestive tract and dealing it out little by little as the cells of the body need it. Back in the chapter on digestion we saw that all the blood that passes through the intestinal tract is gathered up by the portal vein and passes through the capillaries of the liver. It is during this passage through the liver that the sugar is taken out of the blood and stored in the form of a less easily dissolved material known as animal starch or glycogen. The liver cells have the ability to convert sugar into glycogen and they do this whenever the amount of sugar in the blood passing through them is greater than the very small amount which is suitable for the body cells. The blood that leaves the liver carries in it only this small percentage. The liver cells have the ability to change glycogen back into sugar, and this they do whenever the blood that enters them is deficient in it, so that the blood leaving the liver tends always to have the same amount of sugar in it. Whenever it enters with more, there is a conversion of sugar into glycogen; whenever it enters with less, there is a conversion of glycogen back into sugar.

So important is the protection of the body cells against having too much sugar in the fluids surrounding them that the kidney acts to prevent undue accumulation; this it does by withdrawing from the blood and passing out into the urine any sugar that may be in the blood in excess of the small amount which the tissues are able to endure. Thus we see that if the liver did not have its function of converting the sugar into glycogen we would have to change our eating habits completely, taking only a little food at a time instead of eating it in three meals, since otherwise most of our food would be wasted by being passed out from the kidneys as fast as it was poured into the blood from the digestive organs. There is a limit to the ability of the liver to change sugar into glycogen. If the amount in the portal vein at any one time goes above a certain figure, not all will be saved: a part will escape into the blood stream, and since this will raise the percentage, the kidney comes into action and passes it out. In order for this to happen, there must be a very large amount of digested sugar in the small intestine leading to rapid absorption. Since some starchy foods are easier to digest than others some diets are more likely to result in the appearance of sugar in the urine than others. Some of our common foods, notably honey and corn sirup, consist largely of the kind of sugar the body uses. These require no digestion at all, but are ready for absorption as soon as they enter the small intestine. Naturally, if they are eaten in any quantity, they are likely to flood the liver with sugar beyond the amount which it can change to glycogen. Common table sugar and the sugar of milk have to be digested before they are absorbed and so are less likely to flood the liver. It is true, however, that either of them if taken in very large amounts may do this. The digestion of starch goes on much more slowly and so is absorbed more gradually and it is doubtful whether the liver is ever flooded on a starch diet. Since the presence of sugar in the urine is a common indication of diabetes, it is necessary to know that other conditions may bring it about. Obviously, as in the case of an examination for life insurance, a perfectly healthy person might be rejected on account of the presence of sugar in his urine, if it were not that the examining physician knows of this other possibility and is on his guard against it.

The liver ordinarily turns glycogen back into sugar at just the rate necessary to keep the amount in the blood constant. This means that when functional metabolism is going on glycogen is being turned into sugar more rapidly than when the body is quiet. One of the very interesting discoveries of recent years is that in times of strong excitement leading to the outpouring of adrenalin into the blood the rate of change of glycogen into sugar is much increased, so that instead of the usual small amount there is present in the blood a large concentration of sugar. This is evidently advantageous in insuring ample fuel supplies to the muscles at the time of an emergency. It is wasteful in that a large part of the surplus sugar is passed out by the kidneys. The fact is illustrated that in marshaling the bodily functions for meeting an emergency economy is lost sight of.

In addition to these healthy conditions in which sugar may appear in the urine there is the disease diabetes, in which the presence of sugar in the urine is a conspicuous symptom. In diabetes there is a serious disturbance of the whole fuel-supplying mechanism; the liver does not carry on its function of changing sugar to glycogen and glycogen back to sugar as perfectly as it should, and what is of much more importance the muscles which are the chief users of sugar as fuel cannot use sugar in anything like their usual manner. In fact in severe cases they appear to be almost wholly unable to use sugar as fuel. Since the protein from which the nitrogen has been removed classes itself with sugar in this regard, the muscles are thrown back upon fat as their only source of fuel, and this confronts the body with the danger already mentioned that in the burning of fat when little or no sugar is being burned along with it very poisonous products may be formed. Medical investigators have devoted a vast amount of labor to the attempts to find a diet that can be successfully eaten by diabetics. Starches and, almost equally, proteins are not serviceable because they simply flood the tissues with sugar, making an environment which is not good for the cells, and keeping the kidneys busy getting rid of the surplus. Fats are dangerous for the reason just stated. Quite recently real progress has been made by means of the discovery that when the body is living on its own tissue there will be no accumulation of sugar in the body fluids nor outpouring of it from the kidneys. One who is being starved is living on his own tissues and so by simply starving a diabetic his symptoms can be relieved. This is not in itself a very promising expedient, since evidently without food one cannot go on living very long. The point of the treatment is that after starvation has proceeded until the body is actually living on its own tissue it is possible to begin feeding proteins cautiously until little by little a protein diet can be established in which there is little or no indication of surplus sugar. In other words it seems that when the body is compelled to live upon its own tissue it uses proteins efficiently and will then go on using them efficiently when they are supplied to it in the diet.

In an earlier section of the chapter we talked about the amount of protein that the body requires; now we have to take up the matter of the amounts of energy-yielding foods. It is evident that the amounts of these depend upon the amount of metabolism; the total metabolism is made up of the basic metabolism which is steady, shifting little day in and day out, plus the functional metabolism which depends upon how actively the body works. The main functional metabolism is that of the muscles and it is that which varies from day to day. In the case of children there is the additional metabolism of growth for which energy is required and for which food must be eaten. In order to talk intelligently about the use of foods for metabolism we must have a word by which to express a definite amount of it. The word that we use for this is “calory.” Primarily this word stands for a certain amount of energy in the form of heat; since one kind of energy can be transformed into another kind without changing the actual energy value we can use this word as a measure of all kinds of energy and this has become the custom. The calory as a unit of energy means really the amount of energy in the form of heat required to raise the temperature of 1,000 grams of water by 1 degree centigrade. We can translate it into more familiar terms by stating that it equals almost exactly the amount of energy required to raise a weight of 300 pounds to a height of 10 feet, or 30 pounds to a height of 100 feet, or any other combination of weight in pounds multiplied by distance in feet which figures up to 3,000.

The total metabolism, as we said a moment ago, varies greatly day by day because the extent to which we use our muscles is so different. The basic metabolism is very steady and the functional metabolism of the vital processes like breathing, the heartbeat, etc., is also pretty steady, so that the metabolism for one who makes no use of his muscles is fairly uniform. Of course it differs in large people as compared with small, although curiously enough the difference is not proportional to the weight but to the body surface. Just why this is so we do not know. The average figure for the total metabolism of a resting man is about 1,900 calories a day; this includes the motions that are necessary for eating and swallowing food, since life cannot go on indefinitely without making those motions, but supposes that all other activities are done away with. This figure, as we said a moment ago, holds pretty steady day in and day out. To find the total metabolism on any day we have simply to add to it the figure for the energy expenditure from the use of the muscles, which will vary with the amount of work that is done. In figuring this we have to reckon with the fact that the muscles are like other machines in working at what is called in engineering a low efficiency; by that we mean that the amount of energy that can be gotten out in the form of useful work is less than the energy that is actually consumed; there is a waste of energy which takes the form of heat. Our muscles are under ordinary circumstances about twenty per cent efficient, which means that for every calory-worth of actual muscular work we do we use up five calories-worth of fuel; the energy of four wasted calories takes the form of heat and we all know from experience that the amount of heat thus produced inside our bodies warms us up very quickly, when we are using our muscles actively.