Applied physiology : a handbook for students of medicine

Front matter

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Med

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Preface

With the increasing complexity of the medical curriculum, and the growing inclination to specialism amongst teachers, there is a tendency for the student to keep his knowledge in water-tight compartments. Physiology, for instance, is studied in the laboratory, and clinical medicine in the wards, and too often one finds that the student is incapable of applying his scientific knowledge to his clinical work. This is to be regretted, not only because it tends to lessen the interest of the practical study of disease, but because it leads to unsound and unscientific practice. For, beyond all question, the medicine of the future will be based more and more upon a sound knowledge of the normal working of the human body; or, in other words, upon Applied Physiology, and the best physician will be he who combines such knowledge in fullest measure with a wide practical experience at the bedside.

Inability to apply the teachings of physiology leads also to this curious anomaly, that a man may have been taught, and presumably still believe, certain facts as true physiologically, and yet continue to act in his clinical work upon an apparently quite opposite creed. Such an attitude of mind, it need hardly be said, is v

VI

PREFACE destructive of all sound medical thought, and makes for purely empirical practice.

In the present book — the first, so far as the writer knows, of its kind — an attempt has been made to apply physiology to medicine in the same way as anatomy has long been applied to surgery. Seeing that it is not intended to be a substitute for physiological text-books, but merely a companion to them, all descriptions of methods have been omitted, and only the facts of. physiology dealt with, chief emphasis being laid upon those which have a direct bearing upon clinical work : incidentally these bearings are pointed out.

The reader may be surprised at the small size of the book, but when one deals only with the facts of physiology, it is astonishing to find how little space they occupy, and how few of them have as yet any direct practical implications. It will be observed, too, that there is no chapter on the muscular or nervous systems, or on the special senses. These omissions are intentional, for the writer is convinced that most of ‘ nervemuscle ’ physiology, as ordinarily taught to students, is perfectly useless to the physician ; and, as regards the nervous system and special senses, the time is not yet ripe for writing their applied physiology — clinically, one can as yet hardly make use of more than their applied anatomy.

As the book is designed only for students, and not for specialists, few references to original sources have been given ; but the writer feels that it is due to himself to say that these have always been consulted, and that the book really represents a very large amount of

PREFACE vii

research in physiological literature, which has occupied a fair amount of time for some years. He is indebted to Dr. Leonard Hill and Mr. W. Rainey for kindly revising the proofs.

R. H.

Contents

I. THE APPLIED PHYSIOLOGY OF METABOLISM - 1

II. THE APPLIED PHYSIOLOGY OF BODY HEAT - . 49

III. THE APPLIED PHYSIOLOGY OF THE BLOOD AND

Chapter I The Applied Physiology Of Metabolism

The future of medicine undoubtedly depends upon the chemical physiologist. The scalpel, the microscope, and the simpler application of physical and experimental methods have, so far as we can perceive, nearly finished their work, but the investigation of the chemical processes of the body is just begun. The term ‘ metabolism ’ is used to embrace the sum of these chemical processes, and as most diseases are, in the last resort, the expression of a perverted metabolism, it can easily be understood how important a complete knowledge of the subject would be to the physician. Unfortunately, however, we are constantly hampered by our ignorance in this respect. Most of the intimate chemical changes which go on in the organs and tissues are still utterly beyond our ken. Some we guess at or catch glimpses of from afar ; it is only a few that we fully understand. In the present chapter I propose to give a general sketch of metabolism so far as our present knowledge permits, and shall point out as one goes along the application of the facts which have been ascertained to the problems of clinical medicine — so far, at least, as such application is at present possible.

Metabolism has two great purposes to fulfil : (1) the maintenance unimpaired of the substance of the tissues ; (2) the conservation of bodily energy. The former consists in the replacement of the waste of tissue substance which the stress and strain of vital activity entails ; it is therefore essentially anabolic in nature. The conservation of bodily energy, on the other hand, is achieved by the breaking down of food compounds and the liberation of their potential energy in the form of heat and work, and is therefore preponderating^ katabolic.

We may regard these two aspects of metabolism either from the point of view of the total amount of chemical change which takes place (quantitative metabolism), or from that of the intimate nature of the chemical processes which they involve (qualitative metabolism). As the former point of view is the simpler, it will be best to consider it first.

Quantitative Metabolism.*

1. The Conservation of Tissue Substance.

As protein is the only constituent of the food which is able to repair tissue waste, it is obvious that this aspect of metabolism when looked at quantitatively resolves

* In order to appreciate the relative mass of the various tissues which compose the ‘ vital machine ’ and take part in metabolism, it should be remembered that, of the total body-weight, about 40 per cent, is made up of muscle, 5 per cent, of blood, 2 per cent, of brain, whilst the other internal organs and the skeleton make up the remainder.

itself into the question, How much protein is required to make good the daily destruction of tissue? This question is one of such enormous practical importance to the physician that a little space must be devoted to a statement of the prevailing physiological views upon it.

How much Protein is required? — If one attempts to solve this problem by the simple method of estimating the amount of nitrogen excreted daily, one is at once met by the difficulty that * nitrogenous equilibrium ’ can be maintained on very varying quantities of protein. The more protein the food contains, the more nitrogen is excreted, and this goes on until the limits of the digestive capacity are reached. If, again, one takes the amount of nitrogen excreted during fasting as the basis for constructing the diet sheet, it is found that if this amount alone be supplied it is insufficient to maintain equilibrium, and nitrogen is lost from the body. At what point above this, then, is one to fix the proper quantity? Despairing of any experimental solution of this problem, the older school of physiologists fell back on purely empirical observation. They estimated the amount of protein actually consumed by groups of individuals on freely chosen dietaries, and took this as the answer to the question. It was in this way that Yoit arrived at the standard of 118 grammes, which was long accepted as the proper amount of protein for the diet to contain, and it was upon this estimate that the prevailing ‘standard dietaries’ were constructed. Within the last few years, however, physiologists have begun to question the correctness of the Voit standard. Investigation of the diets of many

1—2 individual vegetarians, and of vegetarian races, for example, revealed the fact that health and activity could apparently be maintained on much smaller allowances of protein than the Yoit standard postulated. Finally there came the well - known experiments of Chittenden, which showed for the first time on a sufficiently large scale that, beyond all apparent doubt, health and efficiency could be maintained for indefinite periods upon a much smaller protein intake than 118 grammes. Chittenden fixes the protein optimum, indeed, or the best amount for habitual consumption, at about 60 grammes per day, which is not far above the minimum upon which nitrogenous equilibrium can be maintained. Any intake above this he would regard as a luxus consumption.*

* It might seem that in human milk we would find a standard which would furnish a guide to the proper proportion of protein which the diet ought to contain.

An infant of six months, weighing 6’7 kilogrammes, and consuming an average quantity of breast milk (950 grammes), consumes about 14 grammes of protein, and has a total energy intake of 594 Calories per day. This is about 2 grammes of protein per kilogramme, and if an adult of average weight (70 kilogrammes) consumed the same proportion of protein, he would require 140 grammes in his diet, which is much above even the Yoit standard.

There is, however, another way of looking at the question : Of the total Calories taken in by the infant, fifty-seven are derived from protein ; and assuming the adult to require an intake of 3,000 Calories per day, and that the same proportion of these is derived from protein as in the child, then the amount of protein the adult should consume would only be about 70 grammes per diem, which is approximately Chittenden’s standard. Which is the correct way of looking at the question it is difficult to say. If the adult is to take as much protein per unit of his body-weight as the child, then

If Chittenden be right, there can be no doubt of the far-reaching effect of hia views ; our dietary standards would have to be revised, and vegetarianism would become, not only uninjurious, but a system highly to be commended on grounds alike of humanity, hygiene, and economy. To consume a superfluous quantity of such an expensive food constituent as protein is certainly not only wasteful, but, from the great amount of work required for its digestion and the excretion of its end-products, physiologically injurious as well. Chittenden, indeed, is of opinion that many of the degenerative diseases of later life, as well as many of the paroxysmal neuroses, such as migraine, met with in younger patients, are directly due to a luxus consumption of protein.

Notwithstanding the apparently convincing nature of Chittenden’s experiments, it will be well for the physician to be cautious in applying their results in practice. It is all very well for the physiologist to dismiss contemptuously the universal practice of mankind as having been arrived at purely empirically, and as affording no basis on which to establish rational rules of diet ; but the practical physician, whose art has taught him the safety of a wise empiricism, cannot so lightly discard a rule which has been adopted semper et ubique et ab omnibus.

even the Yoit standard is too low. If, on the other hand, he is to take only the same amount of protein in proportion to the total energy value of his diet as the child, then Chittenden’s standard is correct. It is true that the child is constantly laying up protein in the body in the form of new tissue, but against this has to bo set the fact that the destruction of tissue in the adult is greater, owing to the performance of muscular work.

It may be possible to live in apparent health on a low protein intake for a long time, but can one do so with impunity always ? It may be true that a luxus consumption of protein tends to produce disease, but may an habitually low intake not predispose to diseases of another sort ? To take only two examples, our experience of the treatment of tuberculosis has shown the value of a liberal meat diet in effecting a cure, and it has been found in the Japanese Navy that a liberal protein ration is a prophylactic against the ravages of beri-beri. It may well be, in short, that the supply of a liberal proportion of protein is necessary for the proper production of these chemical compounds, which are one of the means by which the body defends itself against invasion by bacteria. Again, it may prove, when the reserve of protein in the body is low — as it must be on an habitually low intake — that any attack of acute fever will result in too great inroads on the proteins of the fixed tissues, with all the dangers which that entails. These are only some of the considerations which must give us pause in at once applying to practice the results of physiological experiment. Time alone can show whether such objections deserve any weight. Meanwhile every practising physician, by observing how patients who habitually consume little protein resist the attacks of disease and withstand it when attacked, can help to furnish the data which are necessary for a final judgment in the matter.

Storage of Surplus Building’ Material. — In any discussion of the conservation of matter in the body, two influences must be considered : (1) the effect of growth, and (2) the influence of muscular labour. As regards growth, physiology teaches that it can only take place when protein is supplied in excess of the actual needs of the body as measured by the output of nitrogen. In the earlier period of life, therefore, a luxus consumption of protein is actually necessary, a point which must be borne in mind in constructing dietaries for the young. On the other hand, experiment has also shown that, once growth is completed, it is exceedingly difficult to bring about a storage of protein in the body, except when the muscles are being much exercised.* So soon, moreover, as the period of exercise ceases the protein gained is rapidly lost again. One cannot hope, therefore, to make an individual muscularly strong merely by supplying him with protein ; exercise must always be taken at the same time. Convalescence from acute disease is another event which permits of protein being stored up, for in such circumstances the waste of tissue which has taken place is rapidly made good by retaining protein in the body, even although the amount supplied in the diet be not very great. During pregnancy, too, protein is retained in order to take part both in the formation of the foetus and in the hypertrophy of the uterus and mammte.

As regards the influence of muscular labour, experiment has clearly shown that the waste of tissue which it causes is not really great, and there is therefore lo physiological reason for the consumption of an excessive

* The gain of protein in a fully-grown animal is attained by an hypertrophy of cells already existing, not by the development of new cells.

quantity of protein food during training merely to make good wear and tear.

We may now pass on to consider the other great function of metabolism

2. The Conservation of Bodily Energy.

1. Expenditure of Energy.— By means of experiments with the respiration calorimeter the daily expenditure of energy in the body can be calculated without much difficulty. The chief items of expenditure are these : (i.) internal work (heart, respiration, heat production, secretion, excretion, etc.) ; (ii.) digestive work ; (iii.) external or muscular work.

The expenditure of energy in the performance of mental work cannot be calculated, and, indeed, external work is the only item which can be estimated with any accuracy.

(i.) By internal work is meant all the work which is required for carrying on the chief vital processes of the body, without which life would be impossible. The work of the heart alone amounts to about 20,000 kgm. (183 Calories),* or about 64 foot-tons, per day. In diseased conditions, in which the work of the circulation is carried on with difficulty, this amount may be very largely increased (see Chapter IV.), and may become so great, indeed, that no margin of available energy is left for any form of external work. A patient in such a condition is thus inevitably condemned to a life of complete inactivity. The work performed by the muscles of respiration in elevating the chest amounts to about * Allowing for heat produced as well.

6,500 kgm., or 21 foot-tons, daily. Here, again, in pathological conditions this necessary work may be greatly increased in amount, and the daily output of energy of the body proportionately raised.

Of all the items which fall under the heading of internal work, heat production is, however, by far the greatest. The mere performance of work by the heart and the muscles of respiration involves the liberation as a by-product of more than twice as much energy in the form of heat as is actually required for the performance of the mechanical work of respiration and circulation. Whether besides this waste heat an additional quantity is produced as a result of the mere vital activity of the cells (apart from any work done) is a matter on which physiologists are not all agreed, and we shall return to its discussion later on (Chapter II.). The question is an important one ; for if heat results from the mere ‘ living ’ of the cells — if it be a by-product or excretion of life as such — then it is conceivable that cells of a low degree of vitality may produce less heat, and therefore expend less energy, than those which are more ‘ alive,’ and an explanation would be found for the apparently greater degree of economy in metabolism which some individuals exhibit when compared with others. Some such explanation, as we shall see, is almost necessary to account for individual differences in nutrition.

(ii.) The item digestive work covers the expenditure of energy required for the digestion and absorption of the food. It is an item of no small importance, amounting as it does in the case of an ordinary mixed diet to about 150 or 200 Calories — i.e., about G to 8 per cent, of the total intake of energy. It varies, of course, with the hulk of the diet, for the manipulation of large masses of food materially increases the amount of muscular work to be done by the stomach and intestine. This has to be taken into consideration in appraising the value of some forms of purely vegetable diet which are apt to be very bulky. Thus, it has been calculated that in the case of a horse fed entirely upon hay 48 per cent, of the energy of the food is expended in its digestion and absorption. It varies, too, with the chemical constitution of the food. The digestion of proteins seems to demand most work, then that of carbohydrates, whilst that of fats requires least of all. Of common articles of food, milk is that which entails the least digestive work. Anything which increases the frequency or force of peristalsis also raises expenditure under this head, and part at least of the wasting effect of purgation or diarrhoea is thus to be explained.

(iii.) By external (as distinguished from internal) work is meant the work expended by the muscles in performing not only the day’s task of labour, but that required for locomotion and all the thousand ways in which the skeletal as opposed to the visceral muscles are brought into play. It is inevitably, therefore, a heavy item.

It must be remembered, too, that all expenditure on actual external work is unavoidably accompanied by an increase of the wTork done by the heart and lungs as well as by an increased production of heat. Physiologists differ as to exactly how much of the increased expenditure of energy that muscular labour entails goes in the production of the outward task and how much in increased internal work, but one may say approximately that only about one-fifth should be put down to the work itself, and four-fifths to internal expenditure. The human body is therefore rather more economical than the best steam-engine, which gives about 15 per cent, of its total discharge of energy as work, and the rest as heat.

Practice, however, is of great importance in this connection, for a man who is skilled in the performance of any special piece of work certainly does it more economically and with relatively less increase of internal work than one who is unskilled. One illustration will make this clear. It was found by Hueppe, in comparing the expenditure of energy by an untrained town-dweller and an Alpine carrier in climbing, that in the course of a six hours’ ascent the former expended 449 Calories in work and 1,000 Calories in heat, whereas the latter expended 884 Calories in work, and only 565 in heat.

Fatigue, pain, overheating of the body, and being in bad health or ‘out of condition,’ are also all of them factors which increase the ‘ expense ’ of work. A man, for instance, who is tired, footsore, and very hot, will expend more energy in walking a given distance than is really necessary to carry his body over the same space under better conditions.

It must not be assumed from all this that human labour is necessarily cheaper than that of a steam-engine. Quite the contrary, of course, is the case. If, for instance, a labourer works five days a week and earns 20s., of which he spends 5s. upon food, and has a total dally intake of energy of 3,000 Calories, of which 500 are expended upon actual mechanical labour (exclusive of increased internal work), then only one-sixth of the sum expended on food is returned in work — i.e., lOd. worth a week represents the external work done, or 4s. 2d. worth, if one assumes that four-fifths have to be added to this to allow for increased heat production. In other words, only one-fifth of a man’s wages really go to produce work if one regards him as a ‘ hired machine.’ One pound of coal a day consumed in a steam-engine would have produced more than as much work, assuming that the combustion of the coal yields 3,000 Calories of energy, and that the engine converts 15 per cent, of this into work.

It will be obvious from this that the larger part of a man’s earnings must always be spent simply in keeping the machine ‘ alive,’ apart from whether it does any work or not. If, on the other hand, one regards the machine as merely a ‘ peripatetic residence for the soul,’ which haB to be kept both alive and warm, then the only waste in it is the heat given off from the surface, and its efficiency from this point of view is advanced to about 50 per cent, of the energy consumed, which is better than that of any human invention.*

2. Income of Energy. — To balance its expenditure, the body is dependent for its income of energy upon the chemical constituents of the food.t Of these, the pro-

* Thurston, 1 The Animal as a Prime Mover ’ (Smithsonian Report for 1896).

-f It is possible that the body may derive some energy from sources other than food. Radiant heat — e.g., from the sun may certainly be regarded as such a source, and there may conceivably be others of which as yet we know nothing.

teins, carbohydrates, and fats are alone of importance. It is true that small amounts of energy are contained in other ingredients of the diet, such as gelatin, but for practical purposes only the three first named need be considered. The energy yielded by a unit (1 gramme) of each of these constituents in the body, expressed in Calories, is as follows :

Protein ... ... ... 4T Calories.

Carbohydrate -.. ... 4T ,,

Now, the total amount of energy expended by a man of average weight doing a moderate amount of muscular work is something between 2,500 and 3,000 Calories, or the amount which would be produced by the consumption of 1 pound of good coal ; and it follows that, if equilibrium is to be maintained, the total intake in the form of food must equal this sum, and if it does not do so chronic malnutrition and inanition result.

What proportion of the total energy required should be supplied by each food constituent is an important question in practical dietetics.

We have already seen that protein should be regarded as only accidentally a source of energy, its chief use being to replace the loss of nitrogenous material in the tissues, and that by far the largest proportion of the body’s energy must be derived from carbohydrates and fats.

Amount of Carbohydrate and Fat required. — As to the proportion of the total intake of energy which should be supplied by carbohydrates and fats respectively, we have no clear physiological data to guide us. So far as the tissues are concerned, indeed, the physiological teaching appears to be that it is largely a matter of indifference, but that, for the sake of the digestive organs, it is best to derive our energy, not from one or other exclusively, but in some measure from both. From a medical point of view, however, it is not so certain that fats and carbohydrates can replace each other in the diet with indifference. Rickets, for example, is a condition which seems to result from the consumption of a disproportionate amount of carbohydrate and too little fat. There is reason to believe, too, that a diet which contains an excess of carbohydrates may predispose to diabetes by straining the sugar-assimilating functions. On the other hand, when diabetes is once developed, we are compelled to Bupply the greater part of the patient’s energy in the form of fat.

Meanwhile it may be assumed that 50 grammes of fat and 500 grammes of carbohydrate represent the accepted standard for each ingredient ; but it must be borne in mind that these are capable of replacing each other to a large extent, in accordance with the digestive peculiarities of the individual and the opportunities he has of obtaining a supply of each ingredient.

The following balance-sheet, the expenditure side of which is derived from observation of an actual case by Yon Noorden, whilst the income is approximately that of the Chittenden standard, will serve to show in a concise form the practical application of the study of metabolism from its quantitative side. It exhibits a surplus of income over expenditure, and it is assumed that the balance is stored np in the tissues in the form of fat. This is a point, however, which requires a little further discussion.

Balance-sheet of Energy (in Calories).

Man of 56 kilogrammes.

Expenditure. Income.

Calories. Calories.

Kj

1. Internal work (heart,

70 grammes of pro-

respiration, heat

tein = production)

1,550 grammes of fat =

2. Digestive work grammes of carho-

3. External muscular

hydrate ... =

2,050

work —

(a) Actual work

(6) Increase

—

Total

2,630

Total

2,802

Balance, 172 Calories = 17 grammes of fat, or about $ ounce of adipose tissue.

Storage of Surplus Energy. — It is taught by philologists that any surplus of income over expenditure of | energy is inevitably stored up in the form of fat. If, for example, an individual consumed a diet which contains just as much energy as he is expending daily, and we add to this 1 pint of cow’s milk, we shall increase the intake of energy by about 425 Calories. If this be stored, it will form 45 grammes of fat, or 2 ounces of adipose tissue, and the individual’s weight will go up by this amount every day. From the practical point of view, however, the matter would seem not to be so simple as this. One cannot ensure, by merely supplying an excess of income over expenditure of energy, that a patient will put on fat. Experience at the bedside seems to show that there are persons whom it is almost impossible to fatten. What happens to any excess of food supplied in such circumstances it is difficult to say, but in many cases, at least, it appears to fail of elaboration in the liver, and to induce a ‘ bilious attack,’ by which in some way it is got rid of. We shall return to the discussion of this subject, however, again.

The three chief depots in which the surplus energy of the body is stored in the form of fat are, in order of size and importance, as follows : (i.) subcutaneous tissue ; (ii.) abdominal cavity ; (iii.) intermuscular connective tissue. When a man fattens, the subcutaneous and intermuscular depots fill up first, the abdominal cavity more slowly. Conversely, when energy is required by the body to meet a chronic deficit in the intake, the abdominal fat is drawn upon first ,* then the fat below the skin, and after that the intermuscular fat, whilst fat in the organs is only made use of in extreme emergencies.

A deficiency in the total amount of food consumed is more serious than a deficiency of protein alone ; for in the former case both protein and fat are lost by the body, in the latter protein alone. Nor would it seem that the loss of protein, when the total amount of energy in the diet is defective, can be altogether prevented even by consuming large quantities of protein food — e.g., meat.

* The apparent persistence of a large abdomen, in spite of a large reduction in the diet, which may seem to contradict this statement, is really due to the large amount of subcutaneous fat in the abdominal wall, that being a favourite site for its deposition.

A patient on the Banting diet, therefore, must inevitably suffer some loss of muscle besides his loss of fat.