Popular Science Monthly/Volume 57/September 1900/The Human Body as an Engine


By Professor E. B. ROSA.

THERE is no more interesting subject for scientific investigation than the structure and operation, the anatomy and physiology of the human body. That it is an amazingly complex and delicate mechanism, performing a multitude of functions in a wonderfully perfect manner, is, of course, an old story. That in the assimilation of its nourishment and in the growth and repair of its tissue the body obeys the laws of chemistry has long been understood. But that the body obeys in everything the fundamental law of physics, namely, the law of the conservation of energy, has not been so generally recognized. For some years the writer was engaged in some investigations upon this subject.[1] The development of the complex apparatus and unique methods of the research required years of patient labor and study. One of the features of the apparatus was an air-tight chamber, in which a man, as the subject of the experiment, could be confined for any desired period, eating, sleeping, working and living while under minute observation. The experiments usually continued four or five days, but were sometimes prolonged to eight or ten days, and the observations were made and recorded day and night continuously for the entire period.

The atmosphere within the chamber was maintained sufficiently pure to make a prolonged sojourn within its walls entirely comfortable. A current of fresh air, displacing as it entered an equal quantity of air which contained the products of respiration, was maintained continuously. The respired air was analyzed and measured, and the products of respiration from lungs and skin accurately determined. The ventilating air current was maintained by a pair of measuring air pumps, driven by an electric motor. The air was dried, both before entering and after leaving the chamber by freezing out its moisture. This was done by passing it through a refrigerator where its temperature was reduced far below the freezing point. The refrigerator was operated by an ammonia machine, driven by an electric motor. The quantity of air was automatically recorded by the pumps.

The chamber was so constructed and fitted with electrical and other devices as to afford the means of measuring the quantity of heat which the subject of the experiment gave off from his body. And in order to keep the temperature of the room constant this heat was absorbed and carried away by a stream of cold water, the latter flowing through a series of copper pipes within the chamber, and coming out considerably warmer than it entered. So delicate were the regulating devices that the temperature could be maintained constant, hour after hour, to within one or two hundredths of a degree. In some cases the man under investigation worked regularly eight hours a day, the work done being measured by apparatus designed for the purpose.

Food and drink were passed into the chamber three times a day through an air-tight trap. Both were accurately weighed, their temperature recorded and samples reserved for chemical analysis. Solid and liquid excreta were likewise weighed and analyzed. The observations, analyses and computations of a single experiment thus involved a vast amount of labor and expense, which was only justified by the importance of the question under investigation. In order to be able to understand just what this question is, let us see what is meant by the conservation of matter and energy in the physical world.

The impossibility of creating or destroying matter is very generally recognized. Its forms or properties may be altered, chemical and physical changes may be effected, it may, indeed, vanish from sight, but its quantity remains unchanged. Thus ice may turn to water and water to invisible steam, but the total quantity or mass of the substance remains constant; and if by refrigeration the steam be brought to the condition of ice again, there will be precisely the same amount as before. These are physical changes and are easily effected. We simply apply heat to melt the ice and then more heat to vaporize the water. Conversely, withdrawing heat will condense the vapor to water, when a further subtraction of heat will change the water into ice.

Again, wood disappears when burned and seems to be destroyed. And yet we know that the weight of the resulting smoke and ashes is exactly equal to that of the wood. The matter has been changed in form and composition, but its mass cannot be altered. It is not so easy to bring the smoke and ashes into combination again and so restore the matter to its original form as in the case of ice and steam. But this is done by nature. Ashes go to the soil, smoke into the atmosphere. The forces of nature bring these elements together again in plant and tree, and so it comes about that the materials resulting from the burning of wood again become wood, and over and over again the cycle is repeated as time rolls on. Many other examples might be cited to show what is meant by the indestructibility of matter, or the conservation of matter; but these will suffice to show that the one essential fact is that the matter or stuff of a body cannot be destroyed.

Although matter is protean and its transformations limitless, there are certain changes which cannot be made. Iron cannot be turned into silver, nor silver into gold, nor oxygen into nitrogen. There appear to be indeed about seventy or eighty distinct kinds of matter, and so far as we know one cannot be converted into another. They may be united in countless combinations, but each is itself not only indestructible but unchangeable. Why this is so is an interesting subject of speculation. We do not positively know.

That energy is also something which cannot be created or destroyed is not so generally recognized. Transformations of energy from one form to another are constantly occurring before our very eyes; and yet we seldom stop to think what the conservation of energy means in any given case. Energy itself is often defined as that which has the capacity for doing work, and work is done when force or resistance is overcome. A hod carrier does work when, overcoming the force of gravity upon his body and his hod of brick, he climbs to the top of a ladder; and the work done is a measure of the energy expended. Energy stored up in his body has been transferred to the brick in their elevated position, and if they are allowed to fall to the ground their energy is turned into heat, developed by their impact upon the ground. Again, work is done by a windmill in pumping water up into an elevated reservoir, and the so-called 'potential' energy which the water possesses in its elevated position has all been transferred to the water from the wind which drove the mill. If the water be allowed to flow down to the ground again through a water motor the latter could drive machinery and so do work; and the work it could do plus the heat produced by friction would exactly equal the work done in pumping the water up to its elevated position. Thus is the energy conserved, and not destroyed. More or less of it is dissipated by friction, and lost, so far as useful effect may go. But it all remains in existence, somewhere.

Again, coal is burned under the boiler of a steam engine. Heat is produced, steam is generated, the engine does work. The coal possessed a store of energy, potentially. That is, the coal had the capacity of uniting with the oxygen of the air and setting free a store of energy. This energy, potential or latent in the coal, becomes kinetic and evident in the heat of the boiler and the work of the engine. Moreover, the work done by the engine added to the heat given off by the boiler and engine is exactly equal to the total store of energy possessed by the coal. And if from a store of energy, either in the body of a man or horse, or in a pile of wood or coal, a certain portion is expended in doing work, the amount remaining is exactly the difference between that expended and the original amount. In short, energy can be measured, stored up and expended, just as truly as merchandise or money.

Thus the conservation of energy means that energy cannot be created or destroyed; but it may be transferred from one body to another or transformed from one form to another. Heat may be converted into work and work into heat. The chemical energy of a zinc rod may be expended to generate an electric current, and the latter passing through a coil of wire or the filament of a lamp gives up its energy to produce heat and light. The last form of this energy is equal in quantity to the first.

Niagara represents a vast store of energy. Millions of tons of water falling 160 feet could do a vast amount of mechanical work if properly applied through water wheels. More than 50,000 horse power of useful work is actually derived from Niagara's waters, but this is only a small fraction of the total. The energy is, however, given up in falling, even though no useful work is done. In fact, the water is slightly heated by the impact, and the amount of heat produced is exactly equivalent to the mechanical energy lost by the water.

A cannon hall receives a large amount of kinetic energy from the exploded powder as it leaves the muzzle of a great gun. If it be suddenly stopped by a rigid target its mechanical or mass energy is at once converted into heat; that is, into the vibratory motion of the molecules. Ball and target are highly heated. Indeed, lead bullets are often melted by the heat of impact. Meteors living through space come into our atmosphere and their speed is checked by its resistance. Tart or all of their kinetic energy is thus converted into heat. Both air and meteor are heated; heated to so high a temperature that the meteor becomes brilliantly luminous, and we call it a shooting star. The idea of heat due to frictional resistance is common enough. The exact equivalence between the mechanical energy lost and the heat produced is the thing to he especially noticed here.

Let us now take as a final example a locomotive engine. It takes on a store of fuel and water and, directed by its engineer, sets out for a day's duty. The coal to he burned possesses a definite amount of energy. Let us say every pound has one unit of energy, and suppose 5,000 pound of coal are taken. What becomes of these 5,000 units of energy, appearing as heat when the coal is burned?

1. A large amount of heat is required to keep the boiler and engine hot, due to the loss of heat to the atmosphere. The engine cylinders, as well as fire box and boiler, must he kept very hot; other parts of the engine become more or less heated. All parts therefore continually give off heat, and a large part of the heat produced by the burning coal is thus expended.

2. A second portion is expended in doing work. If our locomotive hauls a 500-ton train up a one-per cent, grade for 100 miles it would he doing 2,640,000 foot-tons of work in addition to that required to overcome the friction of the rails and the resistance of the atmosphere. This would require nearly 500 units of energy which would come from the heat of the coal. The work is done through the agency of steam, hut the energy of the steam comes from the burning coal. A small amount of work is also done in pumping water from the tank on the tender into the boiler and in pumping air into the reservoir for the use of the air brakes. This may be called the internal work of the engine. A second portion of the heat is therefore expended in internal and external work.

3. The steam after expanding in the cylinders of the engine escapes into the atmosphere. Although it has been cooled somewhat by expansion, it is still hot, and carries a large amount of heat away with it. Moreover, the smoke and hot air which pass out through the smokestack carry away a large quantity of heat. Hot ashes Likewise carry away heal. Hence a third portion of heat is lost through smoke and steam and ashes. And this is the largest portion of the total quantity of heat generated by the burning coal.

When coal is burned, oxygen of the air unites chemically with the carbon and hydrogen of the coal to form carbonic acid, or carbon dioxid, as it is technically called, and water vapor. The incombustible mineral matter of the coal remains as ashes. Hence smoke contains carbonic acid gas and water vapor in addition to fine particles of unburned coal carried away in the draft of air.

When the grade is steep a great deal of work must be done by the locomotive, much steam is required, and the quantity of fuel burned is large in proportion. When the road is level fuel burns less rapidly, and when the train stops, still more slowly. At night the locomotive rests, fires are hanked and combustion is very slow. This process so briefly and incompletely sketched, is more interesting as one examines it closer, and a locomotive seems almost living when one considers minutely its wonderful performance.

But interesting and instructive though the operation of the locomotive may he, it is not for its own sake that E have mentioned it. It is rather in order to point out a remarkable parallel between its operation and that of a human body. A parallel, indeed, between the operation of a complex inanimate engine of iron and steel, and a still more complex living engine of flesh and hone and blood: both obeying the law of the conservation of energy, as well as the other laws of physics and chemistry.

Consider now a human body as a living engine. That man is more than matter is. of course, conceded. But we here regard only the animal body, guided by the brain as its engineer. The day begins, as with the locomotive, by taking a store of fuel and water, namely, food and drink. Food is not. however, burned in the body in a confined receptacle, like coal in the fire box of an engine, hut is digested, assimilated and distributed through the body by means of the circulating blood. And while some of it goes to repair bodily waste, becoming tissue, other portions are oxidized or burned to produce heat. Non-digestible parts of the food pass away from the body as refuse, like ashes from the fire box of the engine. That the body fat and muscular tissue are also burned, producing heat, is literally true. A hibernating animal keeps his body warm all winter by burning up his autumnal store of body fat. Even a well-fed body is constantly wearing away, or burning away, and hence requires constant repair. Thus we see two distinct functions for food, which should be carefully distinguished.

In the first place, as already indicated, food repairs waste and builds up the body. It makes blood, bone and muscular tissue. Herein we see a departure from the parallel with the steam engine. A locomotive is a machine which runs in a way determined by its builder. But it cannot grow nor repair wear and tear. It requires a whole machine shop pus skilful mechanics to do that. The body, on the other hand, not only runs like a complex mechanism when supplied with energy, but also builds itself up and repairs waste. We express this by saying that it possesses vital force or life, but in just what vital force consists is a matter of speculation and controversy. The raw material which is employed in this work of repairing and building up is found in the food. But not all food can be so utilized. Only those materials which contain nitrogen, the so-called proteids, as lean meat, the casein of milk and gluten of wheat, can be made use of in this most important work of growth and repair.

In the second place, food is the fuel of the body and is just as truly burned as is coal in a furnace. Moreover, the quantity of heat which a piece of meat or a slice of bread yields when burned in the body is just the same as if it had been burned in a stove. Complete combustion yields a definite amount of heat wherever and whatever may be the place and manner of burning. Any kind of food may serve as fuel for the body, but those which consist mainly of sugar, starch and fat, which contain no nitrogen and so cannot build up the body, are used chiefly as fuel. These fuel foods form the bulk of our daily ration, comparatively little being required for purposes of growth and repair.

We are hearing a good deal recently about alcohol as a food. When it is remembered that alcohol contains no nitrogen it will be seen that it cannot serve the first function of food, namely, the purpose of growth and repair. It can, however, serve as fuel food, for when taken into the body in small quantities it is assimilated and burned up, producing the same amount of heat as if burned in a lamp. In sickness this may be beneficial, at times when the body cannot assimilate other foods. But the injurious effects of alcohol upon the digestive and nervous systems are so important and far-reaching that its value as a fuel food sinks into insignificance in comparison.

The process of combustion or burning in the fire box of our locomotive consists, as has been said, in oxygen of the air uniting with the carbon and hydrogen of the coal, forming carbonic acid and water, and setting free a definite quantity of heat for every pound of fuel so burned. So, in exactly the same way, oxygen, which has been taken up by the blood from the air in the lungs, unites with carbon and hydrogen in the tissues of the body and forms carbonic acid and water, yielding precisely the same amount of heat as though the combustion had occurred in a furnace. This idea of food, that it is literally fuel, is a very suggestive one. And as fuels differ in the quantity of ash contained and the amount of heat produced, so food materials differ in the quantity of undigestible residue and in their heat-producing power.

Remembering the analogy of the steam engine, let us now inquire what becomes of the energy supplied to the body in the fuel foods eaten, and which is turned into heat by this process of combustion constantly going on.

1. A large amount of heat is constantly being expended in keeping the body warm. Like the locomotive, the body is warmer than the surrounding air, and is constantly losing heat to the atmosphere. Unlike the locomotive, however, the body has a nearly uniform temperature throughout, namely, 98 degrees Fahr. The delicate regulation of temperature which is automatically maintained in the animal body is one of the wonders of physiology.

2. A second portion of energy is required to do the mechanical work of the body. When a locomotive hauls a loaded train up grade, or steams up grade alone, it is doing work in proportion to the total weight and the height to which it is carried. So when a man walks up hill or climbs a ladder he is lifting his body against the force of gravity, and hence doing work. If his weight be 200 pounds he is doing twice as much work as though he weighed only 100 pounds. If a man weighing 150 pounds climbs Bunker Hill Monument (220 feet), 33,000 foot-pounds of work will then be done; and if he succeeds in making the ascent in one minute, he would be doing work at the rate of one full horse power for that minute. If he climbs a mountain two miles high in three hours and twelve minutes he would be doing work in so lifting his body at the rate of one quarter of a horse power. This is, of course, a faster rate of work than an average man could maintain. In all the functions of daily life the body is necessarily doing some mechanical work. Even dressing and eating require a certain expenditure of energy, and in ordinary business and manual labor the amount of mechanical work done is considerable. Moreover, a large amount of work is done by the heart in pumping the blood through the circulatory system, and by the chest in respiration. This, then, the internal and external work done, as in the locomotive, represents the second portion of energy derived from the food eaten.

3. The warm air, carbonic acid gas and water vapor passing away from the lungs in respiration carry with them a large amount of heat. This corresponds to the loss of heat in the locomotive through the smoke passing out the smokestack, and in both cases the loss is greater when work is being done and less during inaction. The refuse products of the body (as the ashes of the locomotive) also carry away heat. This is the third portion of heat and is a large one.

Work is done in the locomotive by the expanding steam in the cylinders of the engine. The steam is cooled as it expands. Hence heat disappears when work is done; that is, is converted into mechanical energy, and a steam engine is hence called a heat engine; an engine for converting heat into work, according to the law of the conservation of energy. As the pistons are pushed to and fro by the tremendous pressure of the expanding steam, the reciprocating motion is communicated to the great drivers of the engine by strong arms of steel. But how is work done in the body? That is a question of prime importance and of surpassing interest. When muscle contracts and force is exerted, as when the body is lifted or an oar is pulled, muscular tissue (or material stored in muscular tissue) is oxidized; that is, burned, and heat is produced; yet not as much heat appears as would have appeared on the combustion of the same amount of body material if no work had been done. Apparently, then, heat has been converted into work. But we cannot trace the process with the same clearness as in the cylinder of a steam engine. Whether the potential energy of the body material is directly converted into work, or whether combustion first produces heat and a part of this heat is then converted into work, we do not know. In other words, we do not know whether the animal body as a machine for doing mechanical work is a heat engine or some other kind of engine. This is a fundamental question, as well as a very difficult one, and to a student of thermodynamics and physiology it prompts all sorts of speculation.

When one tries to picture to himself how the potential energy of food or body tissue can be directly converted into mechanical work, he is apt to turn to the other alternative and imagine that in some way the body is a heat engine. For we know that heat results from the oxidation of tissue, and we also know how heat can be converted into mechanical work. But we are at once confronted with a difficulty. One of the fundamental laws of thermodynamics requires that when heat is converted into work there shall be a difference of temperature between the source of heat and the place to which the heated material employed passes after doing the work. In other words, in a heat engine, whatever the mechanism, there must be a fall of temperature, which is greater as the relative amount of work, or efficiency, is greater. In the human body the efficiency perhaps surpasses that of the best steam engines; hence there should be a fall of temperature comparable with that between the boiler and condenser of a steam engine. This may be 100 degrees or more, and we do not know of any such difference of temperature in the body. Indeed, we know, on the contrary, that the temperature of the body is remarkably uniform, as already stated. It is possible, however, that there are molecular differences of large amount. In other words, if we could make an ultra-microscopic survey of temperature in a muscle during contraction, there might be found places of high temperature where combustion was occurring, and all the requirements of a heat engine of molecular dimensions fulfilled. But this is a matter of speculation. The process may yet be found to be electrical, or something else quite different from that of a steam engine. We thus find between the animal body and a locomotive engine a striking parallel. In many particulars the chemical and physical processes going on in the latter are found also in the former. In both, the fundamental law of the conservation of energy is strictly observed. Nevertheless, the animal body considered simply as a machine is far more complex in its structure and operation than the engine, and far more of mystery envelops its working. Much remains for the chemist and physicist and physiologist to reveal, and no more fascinating field of research exists.

  1. The work was done at Wesleyan University, in collaboration with Prof. W O. Atwater, under the patronage of the University and the U. S. Department of Agriculture.