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Man's energy requirements
MAN’S
ENERGY
REQUIREMENTS
L. H. NEWBURGH, ALlI.,”
ANN ARBOR, NIGH.
IVING matter depends for its supply of energy solely upon that portion of d the sun’s radiation which strikes the surface of the earth. This type of energy is universally evident as heat and light and is the most common example of kinetic energy. In contrast, energy exists also in the potential form. The latter is not apparent to the senses and its presence can be demonstrated only by converting it to the kinetic type. For example, when coal is burned, that is, oxidized, heat is evolved. Since no new energy can be produced by such chemical reactions, it must be true that the kinetic energy a.ppearing as heat was present in the coal originally in a quiescent state, namely as potent.ial energy. It is equally true that the reaction may go in the opposite direction. Kinetic energy may be changed to potential energy, and this is what takes place in the green plants. In the presence of sunlight, the chlorophyll catalyzes a reaction between carbon dioxide and water, which produces ca.rbohydrates a.nd fat, with the relea.se of oxygen and the conversion of some of the kinetic energy impinging on the green leaf as sunshine to potential energy. The reaction may be expressed in the form of a chemical equation thus: 6 CO, + 6 I-I,0 -+ C, H,, 0, + 6 0,. But this reaction, so simply expressed by chemical symbols, can be accomplished only by living organisms in possession of chlorophyll, hence only by green plants. Animals are quite incapable of converting kinetic to potential energy. Accordingly, they are unable to store any of thle sun’s radiation or to use it in any way directly. The energy displayed by the animal is released from the potential state by oxidation of fat, carbohydrate, and protein. In final analysis, 1Teis completely dependent upon the plant for their formation. These substances, acquired a.s food and stored in the depots, contain the potential energy ready for conversion to the active state according to the needs of the organism. This fundamental process may be expressed as a simple cheE;ical equation thus :, 6CO,+6H,OtC,H,,O,s60,
I
It will be seen that this equation contains exactly the same entities in the same amounts as those appearing in the equation employed to express the reaction that the green plants employ to convert the sun’s kinetic energy to potential energy. But they move in opposite directions with the result t,hat oxygen is released in the first one from carbon dioxide and water and added to carbohydrate in the second one to form carbon dioxide and water. This gives rise to __Read before the Fourth Annual Seminar for th’e Study and Practice of Dental Medicine, The Ahwahnee Hotel, Yosemite, Calif., Oct. 20, 1947. *Department of Internal Medicine, University Hospital, University of Michigan.
the conception that the plant benefits from the animal because the latter produces the carbon dioxide essential t,o the plant, and the animal relies upon the plant to produce the oxygen and stored energy that it must acquire from a source outside of itself. An essential step in the study of the energy available to man was the development of a technique capable of measuring the amount of potential energy residing in the food eaten by him. The bomb calorimeter devised many years ago by Bertellot. is quite satisfactory for this purpose. Briefly, this instrument consists of a metal chamber into which a known amount of the substance under investigation is placed. The lid of the container which is supplied with a spark plug is then screwed into place. Oxygen under pressure is forced into the chamber through a valve and the former is then placed in an insulated can containing a measured amount of water. An accurate thermometer records the temperature of the water, When all is ready, an electric current is passed through the spark plug. The spark ignites the substance within t,he bomb, and the heat evolved by the oxidation passes quickly through the walls of the bomb into the water. By definition, the amount of heat required to raise the temperatnre of one liter of water 1” C. is one kilocalorie. If t,he end products of oxid.ation in the bomb and in the animal are the same, the same amount of heat is produced in both of t,hem for the same amount of the substance. This is the case In the bomb, its nitrogen is comfor carbohydrate and fat but not for protein. pletely oxidized to NO,, but in the animal organism urea is the chief nitrogenous end product,. Urea as its formula, NH,CO.--NH,, indicates, still contains nitrogen and hydrogen which are not combined with any oxygen, and the carbon must unite with another atom of oxygen to complet,e it,s oxidation. Accordingly, the energy residing in the urea must be subtracted from the value obtained for an equivalent amount of protein oxidized in the bomb. This means that only about 70 per cent of the potential ener,T in protein is available to the animal. In order to determine the net energy value of a diet, a uniform sample is oxidized in the bomb, aEd properly prepared samples of the urine and feces are examined by mea,ns of the bomb also. The :residual energy contained in the latt,er two is then subtracted from t,he value obtained for the diet. Such studies have established the familiar figures of 4 calories per gram of pure carbohydrate and protein, and 9 calories per gram of fat. The rat,e at which the animal carries on the oxidations is not speeded up by of oxygen. Other things being equal, the rate will be the increasing the supply same whether he breat,hes normal air containing 21 per cent of oxygen or whether he inhales 100 per cent oxygen. He absorbs from the inspired gas the amount of oxygen required to oxidize stored food at a rate governed by his own coordinating mechanisms. It has been established that t,he oxidation rate of mammals, including man, when they are completely relaxed, is approximately 35 ca,lories per hour per square meter of body surface. This is the basal metabolic rate and because of its uniformity it has become an important standard value. Its usefulness is increased further by correcting it for the small differences related to age and sex. These basal oxidations release the energy involved in the contractions of the heart and the diaphragm and in t,he tone of the voluntary muscles. The
MAN’S
ENERGY
REQUIREMENTS
217
sum of these processes amounts to about 1,500 calories per twenty-four hours, and the average man in the w’hite collar group expends another 1,500 calories in voluntary muscular activity and in the assimilation of food. The physicists define work as motion against gravity or friction. In the case of the animal, work is performed both internally and externally. The contraction of the heart is internal work. Lifting an object is an example of external work. In both cases, kinetic energy is the motive power, but its ultimate escape from the living organism is different in the two cases. All of the energy employed in the performance of internal work is converted to heat within the organism and is lost as such from the body surface. But when external work is done, the energy equivalent of the work leaves the organism in a form other than heat. If man were 100 per cent efficient in the performance of external work, there would be no increase in heat production due to this activity. He would in that case be an ideal machine. But he is very far from that. Under favorable circumstances, he is 25 per cent efficient. In biologic language, this means that four calories must be changed from the potential to the kinetic state by oxidation in order to perform one calorie equivalent of external work. The three other calories appear as heat within the body and increase the heat production and its loss to that extent. Since the mechanical equivalent of energy is known, it is possible to calculate efficiency. This is done by first recording the loss of heat from the man during a period of rest and subsequently while he is performing a known amount of work. The ratio of the energy equivalent of the work to the additional loss of heat gives the efficiency. The mechanical equivalent of energy is expressed quantitatively as: 1 calorie = 427 kilogrammeters. Using more familiar language, it may be stated that one calorie of energy will raise one ton one and one-half feet, provided there is no frictional loss. To perform this amount of work, an efficient man would expend four calories. Clearly, the energy cost of external work is small indeed. For example, a man weighing 190 pounds who is 25 per cent efficient can lift his own. body up two flights of stairs at a cost of only 1 Gm. of fat.. A mile of horizontal walking costs about ‘75 calories, which is the energy contained in S Gm. of fat or one slice of bread. The oxidations do not take place primarily to produce body heat, as is evident from the following facts. A man reclining quietly produces no less heat when exposed to air whose temperature is 104” F. than when it is 75” F. The oxidative rate is the same under both circumstances even though in the first case he is absorbing heat from the environment and can avoid hyperpyrexia only by evaporating large amounts of water. Bodily heat may be regarded as the final state in the transformation of energy. The chemical reactions take place to supply the organism with the energy required for its existence. After this energy has fulfilled its purpose, it is changed into heat in accord with the universal law of nature. Since heat is an inevitable feature of the life process, the highest forms of life have finally developed mechanisms that take advantage of this heat. The mammals have succeeded in establishing a constant internal temperature by the creation of an elaborate system for controlling the loss of heat to the outer world.
Barring abnomai conditions, the outflow of heat keeps pace with its 1)~ duction. Hence; the metabolic rate can be determined by measuring either the loss or the production of heat.. Three types of heat transfer are employed: radiation, conduction, and vaporization of water. Neat flows from. the warmer to t,he cooler region 1,~ radiation and conduction in proportion to the difference in the two temperatures. lVhen the temperature of the skin surface and the surrounding air are the same, no transfer of heat can be effected by either radiation or conduction. This same principle applies to the passage of heat from the interior of the body where it is produced to the surface where it is lost. Heat is removed also by evaporat’ion from the skin and lungs. It requires 0.58 calorie to convert 1 Gm. of water from the liquid to the gaseous state. This energy remains in the vapor as it passes away from the body and is released as the equivalent amount of heat when the gas condenses to liquid water by cont.act with a cool object. The mechanism that controls the loss of heat is in its least active state when the nude recumbent, individual is exposed to quiet air whose temperature is about 84” F. Under t,hese circumstances, the skin temperature is the same over i-he whole body and is about 92” 5’. At this relatively small production of heal-, the interval of about 6” betlveen the internal and surface temperat’ure is sufficient to permit t,he met,abolic heat to reach the surface at its rate of production. From here outward, three-fourths of the heat passes away by radiation and conduction corresponding to a temperature gradient of about 8” between skin and air. The remaining fourth is removed by vaporization. If these conditions continue for twenty-four hours, about 750 Gm. of water will be evaporated fror:1 the lungs and skin. This loss of gaseous water is an entirely passive process dependent upon t,he exposure of a moist protoplasmic mass to a drier air and to their differences in temperature. If now the air temperature is lowered about 10” to 75” F., t,he skin temljeratures which had previously been nearly uniform over the whole body SLUface at approximately 92” 3’. mill show interesting responses. A considerable Pall will occur in the distal regions. For example, the temperat,ure of the great toe will decline from the previous 92” to about 77” F.-a fall of 15”. But, the skin over the trunk will have changed little, about 2” and the internal temperature not at all. The large fall in distal temperatures is caused by constriction of the blood vessels in the extremities. This device reduces greatly the heat loss from the extremities. Since the internal temperature does not fall and the heat production does not increase, it is clear that the rate of heat loss from the body as a whole does not increase when it is exposed to the cooler air. The greater loss from the central regions due to lowering of air temperature without corresponding fall in the temperature of the skin over the trunk is compensated for by the controlled cooling of the arms and legs. The nicety with which this mechanism operates may be appreciated by considering the following facts. A number of persons whose basal heat, production is normal recline unclothed in a room in which the air temperature is kept at SO” F. At the end of an hour, the skin temperature of the grea.t toe is fonnd to be almost iden1,ica.l throughout the series. Next, patients with hyperthproidism arc exposed to the same con
MAN'S ENERGY REQUIREMESTX
219
ditions. Kow it is found that the toe is warmer than in the case of the controls, and that there is a proportionality between the increase in toe temperature and the augmented basal heat production which is so systematic that the basal metabolic rate can be predicted with considerable accuracy from the toe temperature. When the nude resting subject is exposed to air temperatures less than 75” F., a point is reached soon at which more heat must be produced to prevent decline of internal temperature even though the peripheral skin temperature goes down with the air temperature. Shivering is the means of producing more heat. This phenomenon is the expression of a reflex, and the extent of these muscular contractions is proportional to the need for heat up to a maximum at which the heat production is ten to fifteen times the basal rate. When we turn now to the response to air temperatures rising above 85” I?., it is found that less and less heat is lost by radiation and conduction, because the interval between the skin an-d air temperature is lessening. At the same time, the heat lost by vaporization of water is increasing slowly. But soon an abrupt increase in the evaporation of water is recorded. This marks the point at which the sweat glands begin to take part in the removal of heat. Hitherto, the evaporation of water from the body surfaces has been wholly passive. This statement is based on the study of individuals devoid of sweat glands. Water evaporates from these persons in the sa,me amounts as it does from those possessed of the full (quota of weat glands LIP to about 60 Gm. per hour. Conditions that require the evaporation of more water than this to prevent overheating cannot be met by the persons without sweat glands. In normal subjects under the same conditions, overheating is avoided by evaporation of sufficient amounts of water up to a liter or more per hour. For example, it was observed that a man exposed to an air tempera,ture of 104” F., produced 37 calories per square meter per hour, and evaporated 140 Gm. of water. This carried away 81 calories per square meter per hour. Since the air was warmer than the skin, he could not only not lose any heat by radiation and conduction, but also was actually absorbing heat from the environment. The delivery of a sufficient amount of water to the surface by the pumping action of the sweat glands permitted the dissipation of all of this heat so successfully that the internal temperature did not rise, the ma,n remained comfortable, and the skin seemed to be no more moist than normal. Fevers present interesting abnormalities in hea,t regulation. Two distinct types are recognized. One is exemplified by the malarial paroxysm. The onset with the pale, cool skin indicat,es that the flow of blood through the integument is greatly diminished. This reduces the loss of body heat which, however, is not sufficient to satisfy the special conditions. Sharp increase in heat production is obtained by violent shivering until the internal temperature has been raised 5” or more. All this t,ime the patient feels very cold and endeavors to get relief by piling on more covering and hopin, v that the hot wat,er bottles that are put at his feet will comfort him. This sensat,ion of intense cold is, of course, entirely referable to the rapid cooling of the skin since the internal temperature is rising precipitously. After several hours, the discomfort begins to disappear. Es skin becomes flushed and he is now much too warm. This is the beginning
of a rapid outflow of hat which is shortly increased by profuse sweating. In fact, the sweat glands deliver more water to the surface than can be taken up as vapor by the air, and the remainder runs off as liquid water. In a snrprisingly short time, some three to four thousand calories have first been stored in the deep body tissues and then carried away by the rapid circulation of blood to the skin surface, from which they quickly pass outward. The second type of fever is that in which there is a slow increase in internal temperature followed by a plateau during which the deep temperature remains elevated, The rising temperature is caused chiefly by lessened heat loss with moderate increase in heat production but not requiring the intervention of shivering. The plateau is maintained by avoidance of loss of heat in excess of its production. A heat balance has been established at an abnormally high level. The loss of heat is recorded by enclosing the subject in a box whose walls are so constructed that no heat passes across them. The interior of the box contains many feet of coiled metal tubing through which a measured amount of water is pumped. Thermometers record the temperature of the water as it enters and leaves the space. Heat leaves the body surface by radiation and convection and is absorbed by the water. For example, if the temperature of one liter of water passing through the box in one minute ha,s increased 1” C., the water will have absorbed one calorie of heat per minute. At one corner of the box is a pipe that permits the air to be pumped out through a drying agent. The pipe then passes, without any break in continuity, around the outside of the box and enters it again at the opposite corner. Thus, the moisture that evaporates from the subject’s lungs and skin is caught in the drier. Accordingly, its increase in weight is a record of the amount of water vaporized by the subjec.t. Since it requires 0.58 calorie to evaporate 1 Gm. of water, a simple calculation shows how much heat has left the individual by this route. It merely remains to add the heat absorbed by the water that has passed through the coil to the heat of evaporation to obt,ain the total heat loss. Heat production can be estimated by determining how much oxygen wa.s absorbed in a period of time, since oxidations involving one liter of oxygen yield about 4.87 calories. But this very simple calculation does not give an accurate answer because the heat equivalent of a liter of oxygen is not the same when it combines with carbohydrate as when it combines with fat. In the first case, the value is 5.047 calories, in the second, 4.686 calories. This difficulty can be resolved by determining the output of carbon dioxide in addition to the uptake of oxygen. This permits the calculation of the respiratory quotient which is obtained by dividing the number of liters of carbon dioxide by the lit,ers of When oxygen combines with carbohydrate, the quotient is unity, oxygen. I whereas the oxidation of fat yields a quotient of 0.7. By interpolation, the heat; equivalent of the oxygen for any respiratory quotient can be obtained. Kext. the urinary nitrogen is determined in order to calculate how much protein was metabolized and how many calories were yielded by this oxidation. They are subtracted from the total calories and the remainder partitioned between carbbhydrate and fat according to the respiratory quotient. One has now determined t,Clet,otal heat production and t,he amounts of each of the major foodstuffs whose oxidation yielded this heat.
ENERGY
REQUIREMENTS
L. H. NEWBURGH, ALlI.,”
ANN ARBOR, NIGH.
IVING matter depends for its supply of energy solely upon that portion of d the sun’s radiation which strikes the surface of the earth. This type of energy is universally evident as heat and light and is the most common example of kinetic energy. In contrast, energy exists also in the potential form. The latter is not apparent to the senses and its presence can be demonstrated only by converting it to the kinetic type. For example, when coal is burned, that is, oxidized, heat is evolved. Since no new energy can be produced by such chemical reactions, it must be true that the kinetic energy a.ppearing as heat was present in the coal originally in a quiescent state, namely as potent.ial energy. It is equally true that the reaction may go in the opposite direction. Kinetic energy may be changed to potential energy, and this is what takes place in the green plants. In the presence of sunlight, the chlorophyll catalyzes a reaction between carbon dioxide and water, which produces ca.rbohydrates a.nd fat, with the relea.se of oxygen and the conversion of some of the kinetic energy impinging on the green leaf as sunshine to potential energy. The reaction may be expressed in the form of a chemical equation thus: 6 CO, + 6 I-I,0 -+ C, H,, 0, + 6 0,. But this reaction, so simply expressed by chemical symbols, can be accomplished only by living organisms in possession of chlorophyll, hence only by green plants. Animals are quite incapable of converting kinetic to potential energy. Accordingly, they are unable to store any of thle sun’s radiation or to use it in any way directly. The energy displayed by the animal is released from the potential state by oxidation of fat, carbohydrate, and protein. In final analysis, 1Teis completely dependent upon the plant for their formation. These substances, acquired a.s food and stored in the depots, contain the potential energy ready for conversion to the active state according to the needs of the organism. This fundamental process may be expressed as a simple cheE;ical equation thus :, 6CO,+6H,OtC,H,,O,s60,
I
It will be seen that this equation contains exactly the same entities in the same amounts as those appearing in the equation employed to express the reaction that the green plants employ to convert the sun’s kinetic energy to potential energy. But they move in opposite directions with the result t,hat oxygen is released in the first one from carbon dioxide and water and added to carbohydrate in the second one to form carbon dioxide and water. This gives rise to __Read before the Fourth Annual Seminar for th’e Study and Practice of Dental Medicine, The Ahwahnee Hotel, Yosemite, Calif., Oct. 20, 1947. *Department of Internal Medicine, University Hospital, University of Michigan.
the conception that the plant benefits from the animal because the latter produces the carbon dioxide essential t,o the plant, and the animal relies upon the plant to produce the oxygen and stored energy that it must acquire from a source outside of itself. An essential step in the study of the energy available to man was the development of a technique capable of measuring the amount of potential energy residing in the food eaten by him. The bomb calorimeter devised many years ago by Bertellot. is quite satisfactory for this purpose. Briefly, this instrument consists of a metal chamber into which a known amount of the substance under investigation is placed. The lid of the container which is supplied with a spark plug is then screwed into place. Oxygen under pressure is forced into the chamber through a valve and the former is then placed in an insulated can containing a measured amount of water. An accurate thermometer records the temperature of the water, When all is ready, an electric current is passed through the spark plug. The spark ignites the substance within t,he bomb, and the heat evolved by the oxidation passes quickly through the walls of the bomb into the water. By definition, the amount of heat required to raise the temperatnre of one liter of water 1” C. is one kilocalorie. If t,he end products of oxid.ation in the bomb and in the animal are the same, the same amount of heat is produced in both of t,hem for the same amount of the substance. This is the case In the bomb, its nitrogen is comfor carbohydrate and fat but not for protein. pletely oxidized to NO,, but in the animal organism urea is the chief nitrogenous end product,. Urea as its formula, NH,CO.--NH,, indicates, still contains nitrogen and hydrogen which are not combined with any oxygen, and the carbon must unite with another atom of oxygen to complet,e it,s oxidation. Accordingly, the energy residing in the urea must be subtracted from the value obtained for an equivalent amount of protein oxidized in the bomb. This means that only about 70 per cent of the potential ener,T in protein is available to the animal. In order to determine the net energy value of a diet, a uniform sample is oxidized in the bomb, aEd properly prepared samples of the urine and feces are examined by mea,ns of the bomb also. The :residual energy contained in the latt,er two is then subtracted from t,he value obtained for the diet. Such studies have established the familiar figures of 4 calories per gram of pure carbohydrate and protein, and 9 calories per gram of fat. The rat,e at which the animal carries on the oxidations is not speeded up by of oxygen. Other things being equal, the rate will be the increasing the supply same whether he breat,hes normal air containing 21 per cent of oxygen or whether he inhales 100 per cent oxygen. He absorbs from the inspired gas the amount of oxygen required to oxidize stored food at a rate governed by his own coordinating mechanisms. It has been established that t,he oxidation rate of mammals, including man, when they are completely relaxed, is approximately 35 ca,lories per hour per square meter of body surface. This is the basal metabolic rate and because of its uniformity it has become an important standard value. Its usefulness is increased further by correcting it for the small differences related to age and sex. These basal oxidations release the energy involved in the contractions of the heart and the diaphragm and in t,he tone of the voluntary muscles. The
MAN’S
ENERGY
REQUIREMENTS
217
sum of these processes amounts to about 1,500 calories per twenty-four hours, and the average man in the w’hite collar group expends another 1,500 calories in voluntary muscular activity and in the assimilation of food. The physicists define work as motion against gravity or friction. In the case of the animal, work is performed both internally and externally. The contraction of the heart is internal work. Lifting an object is an example of external work. In both cases, kinetic energy is the motive power, but its ultimate escape from the living organism is different in the two cases. All of the energy employed in the performance of internal work is converted to heat within the organism and is lost as such from the body surface. But when external work is done, the energy equivalent of the work leaves the organism in a form other than heat. If man were 100 per cent efficient in the performance of external work, there would be no increase in heat production due to this activity. He would in that case be an ideal machine. But he is very far from that. Under favorable circumstances, he is 25 per cent efficient. In biologic language, this means that four calories must be changed from the potential to the kinetic state by oxidation in order to perform one calorie equivalent of external work. The three other calories appear as heat within the body and increase the heat production and its loss to that extent. Since the mechanical equivalent of energy is known, it is possible to calculate efficiency. This is done by first recording the loss of heat from the man during a period of rest and subsequently while he is performing a known amount of work. The ratio of the energy equivalent of the work to the additional loss of heat gives the efficiency. The mechanical equivalent of energy is expressed quantitatively as: 1 calorie = 427 kilogrammeters. Using more familiar language, it may be stated that one calorie of energy will raise one ton one and one-half feet, provided there is no frictional loss. To perform this amount of work, an efficient man would expend four calories. Clearly, the energy cost of external work is small indeed. For example, a man weighing 190 pounds who is 25 per cent efficient can lift his own. body up two flights of stairs at a cost of only 1 Gm. of fat.. A mile of horizontal walking costs about ‘75 calories, which is the energy contained in S Gm. of fat or one slice of bread. The oxidations do not take place primarily to produce body heat, as is evident from the following facts. A man reclining quietly produces no less heat when exposed to air whose temperature is 104” F. than when it is 75” F. The oxidative rate is the same under both circumstances even though in the first case he is absorbing heat from the environment and can avoid hyperpyrexia only by evaporating large amounts of water. Bodily heat may be regarded as the final state in the transformation of energy. The chemical reactions take place to supply the organism with the energy required for its existence. After this energy has fulfilled its purpose, it is changed into heat in accord with the universal law of nature. Since heat is an inevitable feature of the life process, the highest forms of life have finally developed mechanisms that take advantage of this heat. The mammals have succeeded in establishing a constant internal temperature by the creation of an elaborate system for controlling the loss of heat to the outer world.
Barring abnomai conditions, the outflow of heat keeps pace with its 1)~ duction. Hence; the metabolic rate can be determined by measuring either the loss or the production of heat.. Three types of heat transfer are employed: radiation, conduction, and vaporization of water. Neat flows from. the warmer to t,he cooler region 1,~ radiation and conduction in proportion to the difference in the two temperatures. lVhen the temperature of the skin surface and the surrounding air are the same, no transfer of heat can be effected by either radiation or conduction. This same principle applies to the passage of heat from the interior of the body where it is produced to the surface where it is lost. Heat is removed also by evaporat’ion from the skin and lungs. It requires 0.58 calorie to convert 1 Gm. of water from the liquid to the gaseous state. This energy remains in the vapor as it passes away from the body and is released as the equivalent amount of heat when the gas condenses to liquid water by cont.act with a cool object. The mechanism that controls the loss of heat is in its least active state when the nude recumbent, individual is exposed to quiet air whose temperature is about 84” F. Under t,hese circumstances, the skin temperature is the same over i-he whole body and is about 92” 5’. At this relatively small production of heal-, the interval of about 6” betlveen the internal and surface temperat’ure is sufficient to permit t,he met,abolic heat to reach the surface at its rate of production. From here outward, three-fourths of the heat passes away by radiation and conduction corresponding to a temperature gradient of about 8” between skin and air. The remaining fourth is removed by vaporization. If these conditions continue for twenty-four hours, about 750 Gm. of water will be evaporated fror:1 the lungs and skin. This loss of gaseous water is an entirely passive process dependent upon t,he exposure of a moist protoplasmic mass to a drier air and to their differences in temperature. If now the air temperature is lowered about 10” to 75” F., t,he skin temljeratures which had previously been nearly uniform over the whole body SLUface at approximately 92” 3’. mill show interesting responses. A considerable Pall will occur in the distal regions. For example, the temperat,ure of the great toe will decline from the previous 92” to about 77” F.-a fall of 15”. But, the skin over the trunk will have changed little, about 2” and the internal temperature not at all. The large fall in distal temperatures is caused by constriction of the blood vessels in the extremities. This device reduces greatly the heat loss from the extremities. Since the internal temperature does not fall and the heat production does not increase, it is clear that the rate of heat loss from the body as a whole does not increase when it is exposed to the cooler air. The greater loss from the central regions due to lowering of air temperature without corresponding fall in the temperature of the skin over the trunk is compensated for by the controlled cooling of the arms and legs. The nicety with which this mechanism operates may be appreciated by considering the following facts. A number of persons whose basal heat, production is normal recline unclothed in a room in which the air temperature is kept at SO” F. At the end of an hour, the skin temperature of the grea.t toe is fonnd to be almost iden1,ica.l throughout the series. Next, patients with hyperthproidism arc exposed to the same con
MAN'S ENERGY REQUIREMESTX
219
ditions. Kow it is found that the toe is warmer than in the case of the controls, and that there is a proportionality between the increase in toe temperature and the augmented basal heat production which is so systematic that the basal metabolic rate can be predicted with considerable accuracy from the toe temperature. When the nude resting subject is exposed to air temperatures less than 75” F., a point is reached soon at which more heat must be produced to prevent decline of internal temperature even though the peripheral skin temperature goes down with the air temperature. Shivering is the means of producing more heat. This phenomenon is the expression of a reflex, and the extent of these muscular contractions is proportional to the need for heat up to a maximum at which the heat production is ten to fifteen times the basal rate. When we turn now to the response to air temperatures rising above 85” I?., it is found that less and less heat is lost by radiation and conduction, because the interval between the skin an-d air temperature is lessening. At the same time, the heat lost by vaporization of water is increasing slowly. But soon an abrupt increase in the evaporation of water is recorded. This marks the point at which the sweat glands begin to take part in the removal of heat. Hitherto, the evaporation of water from the body surfaces has been wholly passive. This statement is based on the study of individuals devoid of sweat glands. Water evaporates from these persons in the sa,me amounts as it does from those possessed of the full (quota of weat glands LIP to about 60 Gm. per hour. Conditions that require the evaporation of more water than this to prevent overheating cannot be met by the persons without sweat glands. In normal subjects under the same conditions, overheating is avoided by evaporation of sufficient amounts of water up to a liter or more per hour. For example, it was observed that a man exposed to an air tempera,ture of 104” F., produced 37 calories per square meter per hour, and evaporated 140 Gm. of water. This carried away 81 calories per square meter per hour. Since the air was warmer than the skin, he could not only not lose any heat by radiation and conduction, but also was actually absorbing heat from the environment. The delivery of a sufficient amount of water to the surface by the pumping action of the sweat glands permitted the dissipation of all of this heat so successfully that the internal temperature did not rise, the ma,n remained comfortable, and the skin seemed to be no more moist than normal. Fevers present interesting abnormalities in hea,t regulation. Two distinct types are recognized. One is exemplified by the malarial paroxysm. The onset with the pale, cool skin indicat,es that the flow of blood through the integument is greatly diminished. This reduces the loss of body heat which, however, is not sufficient to satisfy the special conditions. Sharp increase in heat production is obtained by violent shivering until the internal temperature has been raised 5” or more. All this t,ime the patient feels very cold and endeavors to get relief by piling on more covering and hopin, v that the hot wat,er bottles that are put at his feet will comfort him. This sensat,ion of intense cold is, of course, entirely referable to the rapid cooling of the skin since the internal temperature is rising precipitously. After several hours, the discomfort begins to disappear. Es skin becomes flushed and he is now much too warm. This is the beginning
of a rapid outflow of hat which is shortly increased by profuse sweating. In fact, the sweat glands deliver more water to the surface than can be taken up as vapor by the air, and the remainder runs off as liquid water. In a snrprisingly short time, some three to four thousand calories have first been stored in the deep body tissues and then carried away by the rapid circulation of blood to the skin surface, from which they quickly pass outward. The second type of fever is that in which there is a slow increase in internal temperature followed by a plateau during which the deep temperature remains elevated, The rising temperature is caused chiefly by lessened heat loss with moderate increase in heat production but not requiring the intervention of shivering. The plateau is maintained by avoidance of loss of heat in excess of its production. A heat balance has been established at an abnormally high level. The loss of heat is recorded by enclosing the subject in a box whose walls are so constructed that no heat passes across them. The interior of the box contains many feet of coiled metal tubing through which a measured amount of water is pumped. Thermometers record the temperature of the water as it enters and leaves the space. Heat leaves the body surface by radiation and convection and is absorbed by the water. For example, if the temperature of one liter of water passing through the box in one minute ha,s increased 1” C., the water will have absorbed one calorie of heat per minute. At one corner of the box is a pipe that permits the air to be pumped out through a drying agent. The pipe then passes, without any break in continuity, around the outside of the box and enters it again at the opposite corner. Thus, the moisture that evaporates from the subject’s lungs and skin is caught in the drier. Accordingly, its increase in weight is a record of the amount of water vaporized by the subjec.t. Since it requires 0.58 calorie to evaporate 1 Gm. of water, a simple calculation shows how much heat has left the individual by this route. It merely remains to add the heat absorbed by the water that has passed through the coil to the heat of evaporation to obt,ain the total heat loss. Heat production can be estimated by determining how much oxygen wa.s absorbed in a period of time, since oxidations involving one liter of oxygen yield about 4.87 calories. But this very simple calculation does not give an accurate answer because the heat equivalent of a liter of oxygen is not the same when it combines with carbohydrate as when it combines with fat. In the first case, the value is 5.047 calories, in the second, 4.686 calories. This difficulty can be resolved by determining the output of carbon dioxide in addition to the uptake of oxygen. This permits the calculation of the respiratory quotient which is obtained by dividing the number of liters of carbon dioxide by the lit,ers of When oxygen combines with carbohydrate, the quotient is unity, oxygen. I whereas the oxidation of fat yields a quotient of 0.7. By interpolation, the heat; equivalent of the oxygen for any respiratory quotient can be obtained. Kext. the urinary nitrogen is determined in order to calculate how much protein was metabolized and how many calories were yielded by this oxidation. They are subtracted from the total calories and the remainder partitioned between carbbhydrate and fat according to the respiratory quotient. One has now determined t,Clet,otal heat production and t,he amounts of each of the major foodstuffs whose oxidation yielded this heat.