364 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



starvation be endured, and if water is consumed freely the evil effects of star- 

 vation, as well as the disagreeable sensations of hunger, are very much 

 reduced. 



Potential Energy of Food. — The chemical changes occurring in the body 

 are accompanied by a transformation of chemical energy to different forms — 

 for example, to heat, electricity, and mechanical work. By far the most of this 

 energy takes the form, directly or indirectly, of heat Even when the muscles 

 are apparently at rest we know that theyare undergoing chemical changeswhich 

 give risetoheat. When :t muscle contracts, the greater part (four-fifths) of th< 

 energy liberated by the chemical change takes the form of heat ; a much smallei 

 part (al)ont one-fifth as a maximum) may perform mechanical work, which 

 in turn, as in the case of the respiratory muscles and the heart, may be con- 

 verted to heat within the body. Roughly speaking, an adult man gives off 

 from his body in the course of twenty-four hours about 2,400,000 calories of 

 heat (1 calorie = the heat uecessary to raise 1 cubic centimeter of water 1° C). 

 This supply of heat is derived from the metabolism or physiological oxidation 

 of the proteids, the fats, and the carbohydrates that we take into the body in 

 our food. By means of the oxygen absorbed through the lungs these substances 

 are burnt, with the formation of C0 2 , H 2 0, and urea or some similar nitrog- 

 enous waste product. In the long run, then, the source of body-energy is found 

 in the potential energy contained in our food. Our energy-yielding foods — 

 proteids, fats, and carbohydrates — are more or less complex bodies that are 

 built up originally by plant organisms with the aid of solar energy; when 

 they are burnt or otherwise destroyed, with the formation of simpler bodies 

 (such as C0 2 or H 2 0), their so-called potential energy is liberated in the 

 form of heat, and this is what occurs in the body. From the standpoint of the 

 law of conservation of energy it is easy to understand that the amount of 

 available energy in any food-stuff may be determined by burning it outside the 

 body and measuring the quantity of heat liberated. If a gram of sugar is 

 burnt, it is converted to C0 2 and FLO and a certain quantity of heat is liber- 

 ated ; if the same gram of sugar had been taken into the body, it would event- 

 ually have been reduced to the form of C0 2 and H 2 0, and the total quantity 

 of heat liberated would have been the same as in the combustion outside the 

 body, although the destruction of the sugar in the body may not be a direct, 

 but an indirect, oxidation; that is, the oxygen may first be combined with sugar 

 and other food-stuffs to form a complex molecule which afterward dissociates 

 into simpler compounds similar to those obtained by direct oxidation, or there 

 may be first a dissociation or cleavage followed by oxidation of the dissociation 

 products. In determining the total energy given to the body we need only 

 consider the form in which a substance enters the body and the form in which 

 it is finally eliminated. In the case of proteids the combustion in the body is 

 not so complete as it is outside ; the chief final products are C0 2 , H 2 0, and 

 urea. The urea, however, .-till contains potential energy which may be lib- 

 erated by combustion, and in determining the energy of proteid available to 

 the body, that which is lost in the urea must be deducted. As a matter of 

 fact, it is possible that the proteid in the body is completely oxidized to C0 2 , 



