450 TEXT-BOOK OF PHYSIOLOGY 



creased. Under these circumstances there will be an increase of from 10 to 

 14 per cent, thus raising the heat-production and dissipation per kilogram of 

 body-weight from approximately 32 Calories to 35.5 or 36.5 or a total of 

 2485 to 2555 Calories. The heat liberated by the metabolism during inan- 

 ition is not sufficient to maintain the body in heat equilibrium and there- 

 fore, an increase in the food supply of from 10 to 14 percent, beyond that 

 metabolized is necessitated. A reason assigned for this^ necessity is as 

 follows: During digestion and in the earlier stages of assimilation, the foods 

 undergo a reduction and cleavage, in consequence of which a portion of their 

 contained energy is liberated as free heat which however cannot be utilized by 

 tissue cells in the performance of their various activities. This heat is 

 probably directly dissipated in an environment of 33C. The energy that 

 cells require for the manifestations of their physiologic activities must 

 apparently be derived only from the metabolism of food materials within 

 themselves. Therefore, food materials in amount just sufficient to replace 

 the amount metabolized in inanition, would furnish an amount of heat 

 which would be insufficient to maintain the body in heat equilibrium. 

 Though this holds true for a man weighing 70 kilograms it is not strictly 

 true for men who weigh less or have a different skin area. It has been 

 pointed out by Rubner that the number of Calories dissipated per kilogram 

 of body-weight varies with the weight and the skin area: that the metabolism 

 is proportional to the surface area or, in other words, whatever the weight, 

 there will be for a given surface area, a certain heat loss leading in turn to a 

 given metabolism of food materials. In the following table the weights of 

 different men, their skin areas, total heat-production as well as Calories per 

 kilogram while doing light work are presented. (Rubner.) 



Weight 

 in Kg. 



80 



7 



60 



50 



40 



In the foregoing table it will be observed that in the man weighing 80 

 kilograms, the ratio of skin area to kilogram is 0.0285, while in the man 

 weighing 40 kilograms the ratio is 0.0359. The heat liberated by this slight 

 increase in skin area amounts to 9.4 calories per kilogram weight. 



3. During Work. The foregoing estimates as to the amounts of heat 

 produced have reference mainly to the body in repose. When the body 

 passes into a state of muscle activity, there is at once a notable increase 

 in heat-production in consequence of the increase in the activity of the 

 chemic changes which underlie body activity, as shown by the increase in 

 the consumption of oxygen and the production of carbon dioxid. Not 

 all of the potential energy set free, however, appears as heat; for, if the 

 muscles are engaged in doing work a part of the energy, which would other- 

 wise manifest itself as heat is converted into mechanic motion. From the 

 work done during a period of eight hours it has been estimated that about 

 500 calories are so transformed or utilized. Him calculated from an aver- 

 age of five experiments that a man weighing 67 kilos in repose produced 

 154.4 calories per hour and absorbed 30.7 grams of oxygen per hour; but 



