Physiology: The Science of the Body

by Ernest G. Martin

The sweat glands, as we have already seen, make up part of the service of the removal of waste in that they help to carry away water from the body. To a very slight extent, but much slighter than is commonly believed, they get rid also of waste products of cell activity; for example ammonia, which is one such product, can frequently be detected in the sweat by its odor; practically, however, the importance of sweat glands is not in their discharge of waste organic substances, but only in their discharge of water. In this they are playing a very important part in the regulation of body temperature, so that the further account of the sweat glands as well as of the skin of which they form a part is left for the chapter in which the regulation of temperature is described.

CHAPTER XVIII
MORE ABOUT THE USE OF FOOD BY THE BODY

WE have now brought our description of the various things that happen in our bodies up to the point where we may begin to make some kind of summary of them; particularly in respect to what goes on within our individual cells. We have seen how some of our cells are muscle cells by which we make motions; others are the cells of sense organs by which we get the necessary information for guiding our activities; still others are in the nervous system and carry on the adjustments by which we act in accordance with the indications of our surroundings and also of our past experience; finally we have the cells which manufacture materials, as in the various digestive glands or in the glands which make hormones. We have also looked into the kind of materials which these cells require, where they come from, and how they are prepared in the digestive tract, and we have seen how these prepared materials and oxygen are conveyed to the different parts of the body where the cells can get them and finally how the waste products which all these cells give off are gotten rid of. We are now to turn to the process of metabolism itself as it takes place within the cells. In Chapters III and IV we looked into the use of food for power development and for repairing the wastage of protoplasm, as well as for the making of new protoplasm in growth. We have seen also that the body contains much nonliving material, as in the bones and teeth, which must come from the food and which must be put in place as the result of metabolism on the part of living cells.

The first thing which we wish to take up here is the use of protein in the repair of protoplasmic wastage and in growth. We saw in Chapter III that protein is manufactured originally by the living cells of green plants. We have also seen that protein is the only material that can repair protoplasmic wastage or that can make new protoplasm. We have omitted to say thus far that the most important place from which we can obtain protein is from living protoplasm. It is true that most seeds store up within themselves nonliving protein to be used by the young sprout as it forms, and seeds make up a large part of our diet; but except in grain and other seed foods we obtain our supplies of protein by eating protoplasm. This protein is to be used by us for repairing our protoplasmic wastage or, in parts of us that are growing, for making new protoplasm. We have already seen that the protein which we eat must go through a process of digestion before we can use it for these purposes, and our present task is to explain just why this is necessary and to show how the protein is actually used in our bodies.

A thing about protein which fits it specially to be the chief material of living protoplasm is that it is very much the same sort of substance wherever we find it and yet can differ enough to account for the differences that exist among animals and plants. In spite of the fact that protoplasm analyzes about the same, no matter where it comes from, we are bound to believe that the difference between a dog and an oak tree is at bottom a chemical difference; they are unlike because the protoplasm of one is not the same substance chemically as the protoplasm of the other, and the difference is a difference in the proteins. We can come even nearer home than that and say that the differences between the races of mankind are probably chemical differences between their proteins. Human protein is undoubtedly different from the protein of beef or pork or mutton. What we have then is a substance which can be at the same time similar and different; also since we can make human protein out of beef protein or any other kind which we happen to eat it must be fairly easy to change one into the other.

Protein is about the most complex substance that we know anything about; it is made up of a number of organic acids combined chemically. These organic acids all contain nitrogen, which puts them into a class to which is given the name of amino acids. To the chemist the name amino acid shows a certain kind of chemical formation; to us it need mean no more than an organic acid which contains nitrogen. The proteins which are in our bodies are as complex as any that exist and some of them are made up of as many as eighteen different amino acids. The same eighteen acids are present in the proteins of all the higher animals. When we eat lean beef or pork we get exactly the same eighteen amino acids that are in our own proteins, but not put together in precisely the same way. What we have to do with these proteins is to break them up into the amino acids of which they are composed and then put these together again in the combination which makes up human protein. The breaking up of the proteins is carried on in the digestive organs; we have said a good deal about it in Chapter XV; what were called in that chapter digestion products of proteins we now see are amino acids. These are taken up into the blood stream, carried around the body to the tissue fluids and by the living cells taken up to be built into human protein. Exactly the same thing happens to any proteins that we eat.

One of the great differences between animal protein and plant protein is that the percentages of the different amino acids are very different. Some of the amino acids that make up a large proportion of animal protein are very scantily represented in plant protein. To make human protein we must not only have all the amino acid ingredients, but we must also have enough of every one. On a purely vegetarian diet to get enough of those amino acids which are scantily present in plants we have to eat a large surplus of those amino acids which are specially abundant. In this respect plants as providers of amino acids are less economical than animals, because animal proteins have more nearly the same proportions of the different amino acids as do our own human proteins. Of course, as we will see at once, in theory cannibalism is the most economical way of getting protein; if we were to eat human protein we would have exactly the correct proportion of the different amino acids and so could get along with a minimum amount. This is not to be interpreted as an argument for the practice of cannibalism among human beings, although we may as well face the fact that there is no physiological or dietary reason for avoiding the practice. In some of the lower animals, particularly in rats, cannibalism is a regular part of the life habit. Rats do not have the instinct of storing up food supplies as do squirrels and some other kinds of animals. When food is abundant they multiply very rapidly and then when food becomes scarce the stronger feed upon the weaker. It is largely for this reason that the unsanitary and extremely expensive rat nuisance is so hard to abate.

The amino acids that are circulating in the blood stream after every meal are primarily to be used for repairing protoplasmic wastage; also they serve for the manufacture of new protoplasm, provided growth is going on. In theory an adult who is through with all his growth except in the skin and one or two other minor tissues should be able to get along with just the amount of protein which will make good his protoplasmic wastage. Since protein is an expensive food and likely to be hard to get in times of scarcity, the question of how much protein should be eaten is of great practical importance. There are several ways of studying the problem; one is by the actual study of diets to find out how much protein people do habitually eat; another is by finding out how much the daily protoplasmic wastage amounts to. If no more protein is being eaten than is necessary for the protoplasmic wastage, these two figures should be about the same. The way of finding out how much protoplasmic wastage there is is to go on a diet which contains abundant starch and fat for energy supplies, but no protein or amino acids. When one is on that kind of diet he knows that he will not have to burn up any of his own tissues to supply him with the energy for his metabolism; whatever breakdown of protoplasm occurs on such a diet is the natural wastage of the body and not the result of using tissues for fuel, as is the case in complete starvation. It is fairly easy to keep track of the decomposition of protein in the body, because protein contains nitrogen and the nitrogen is given off almost wholly in the waste products that are passed out from the kidneys. By collecting the urine and analyzing it for nitrogen the amount of protoplasmic breakdown in the body can be determined, provided no nitrogen-containing compounds were taken in with the food. Otherwise, of course, one could not be sure that nitrogen appearing in the urine had actually come from the wastage of protoplasm. The fact is that when an average-sized human being goes on a diet which contains no protein, but is ample in other respects, he loses daily from his body about an ounce of protein. This is proven by the occurrence in the urine of an amount of nitrogen which stands for that much protein. The person may be gaining or losing weight meanwhile; if his consumption of fats, starches, and sugars is excessive, he may deposit some fat, in which case he might gain weight in spite of the loss of some of his actual living protoplasm. Usually though, in experiments of this kind, there is a steady loss of weight made up of the ounce of protein and of three or four ounces of water. We have to remember that living protoplasm is three-fourths or more water, so that whenever any protoplasm wastes away, some water will be lost as well as protein. It is a very interesting fact that this protoplasmic wastage goes on steadily at the rate of about an ounce a day whether the body is active or inactive; this means that the wastage is a matter of the basic metabolism and not of the functional metabolism. The former goes on all the time day and night, in sleep and in waking, and in connection with it the living protoplasm shows this small amount of wastage. Functional metabolism does not, at least under ordinary conditions, increase the amount. Speaking of the body as though it were a machine, we would say that it rusts out just as fast as it wears out. This is one of the features in which the living machine differs from most mechanical devices of human manufacture.

Although the loss of protein due to wastage is only about an ounce a day, nobody can get along on a diet which contains no more than that amount. Between three and four ounces of protein is the average daily consumption of adults in this country. We should not forget that our diet consists of meat, bread, vegetables, fruits, etc., which are mixtures of proteins with the other food materials and with a large percentage of water, so that in order to get three or four ounces of protein we have to eat four or five times that weight of ordinary foodstuffs. There has been much debate as to whether it is necessary or even desirable for adults to eat three or four times as much protein as the body requires for making up its wastage. The decision will have to rest in part on what use the body makes of the surplus. Since from time immemorial human beings have habitually eaten every day this large surplus, it is evident that they have been wasting enormous amounts of good food or else that some use is made of it even though it does not serve its purpose of repairing the body waste. The surplus materials are present in the body in the form of amino acids, since what the cells do in repairing their wastage is to take up from the whole quantity of amino acids in the tissue fluids as much as they require for making good their loss. The mixture of amino acids that is left over will make perfectly good fuel provided the nitrogen that is in it is gotten rid of, and this is what happens in the body. All the amino acids in excess of the amount needed for restoring the tissues are decomposed in such a way that the nitrogen is abstracted in the form of ammonia and the substance that is left, which is a starchlike compound, joins with the other starch products and fats to be burned in the body as a source of energy. We do not know certainly which tissues have the power of decomposing the surplus amino acids. At the present time it is believed that all or nearly all of them can do it, so that as they take up from the tissue fluids the particular amino acids which they need for making good their own wastage they take up also the surplus which they decompose, utilizing the starchlike part for fuel and turning the ammonia back into the body fluid as a waste product.

Ammonia is a very poisonous substance and it quickly poisons the body if allowed to accumulate in the tissue fluids. This is prevented by the action of the liver in changing the poisonous ammonia into a harmless substance known as urea. This urea is carried by the blood stream from the liver to the kidneys where it is passed out to become the chief organic substance in urine. The more protein one eats the more surplus amino acids will there be, and so the more urea will be formed and passed out of the body. Flesh-eating animals and men (Eskimos for an example of the latter) eat a very large surplus of proteins, the fuel for their metabolism being furnished almost altogether either from the usable remains of the decomposed amino acids or the fats that were in the flesh they ate. Some people have been inclined to believe the production of so much ammonia and its subsequent conversion into correspondingly large amounts of urea to be injurious. As a matter of fact, there is no particular reason for thinking this to be the case; it is part of the duty of the liver to change all the ammonia that comes to it to urea and of the kidneys to pass out all the urea that comes to it; so long as these organs are healthy they are able to fulfill these duties effectively, so this does not seem to be a good reason for cutting down the percentage of meat in the diet. It is generally believed that meat has special effects upon the nervous system, such as to incite to cruelty and bloodthirstiness. There is no real scientific proof as to whether this is true or not. The scientific fact is that man is fitted for a mixed diet, neither exclusively of flesh nor exclusively vegetarian. He has lived for thousands of years on that kind of diet and can apparently go on for thousands of years more. We need to remember that the various dietary fads which come into great prominence from time to time are rarely based on a well-established scientific foundation nor have any of them any long experience back of them. On the other hand, the common mixed diet which all of us eat in accordance with custom and the dictates of our appetites has the sanction of thousands of years of successful maintenance of the human race. It is quite true that one can get along on almost any kind of a diet provided it contains enough protein to make good the daily body wastage and enough fuel material to provide for the demands of metabolism. Anyone who is disposed to adopt for himself a dietary fad will rarely suffer seriously from it; on the other hand, those who prefer to eat as our fore-fathers have eaten need not feel conscience-stricken because there is agitation against the commonly accepted diet.