582 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



food and drink and directly from external sources, such as the sun's rays, that 

 these sources may be disregarded. This potential energy of food may be 

 converted into heat directly or indirectly ; directly, as an immediate result of 

 chemical decomposition ; and indirectly, by mechanical movements, such as 

 muscular contraction, the flow of the blood, the friction of the joints, etc. 

 About 90 per cent, of the heat of the organism results directly from chemical 

 decompositions, and about 10 per cent, results indirectly from mechanical 

 movements. The potential energy of the food is transformed into kinetic 

 energy (heat and work) essentially by processes of oxidation. The energy- 

 yielding food-stuffs enter the body in the form of proteids, fats, and carbo- 

 hydrates. The proteid is oxidized into urea, CO 2 , H 2 O, and various extrac- 

 tives ; and the fats and carbohydrates are reduced to CO 2 and H 2 O. During 

 these oxidative processes, by which the potential energy of the molecules is 

 transformed into kinetic energy, the total amount of energy evolved by the 

 complete oxidation of a given amount of any substance is the same whether 

 the processes are carried at once to the final stages, that is, to the final disin- 

 tegration products, or whether they pass through an indefinite number of 

 intermediate stages, provided that the final product or products are the same. 

 In other words, the amount of heat evolved by the oxidation of 1 gram of 

 proteid into urea, CO 2 , and H 2 O is the same when the molecule is oxidized 

 immediately into these substances as when the decomposition is carried through 

 a number of intermediate stages. Similarly 1 gram of carbon oxidized into 

 CO 2 , or 1 gram of H oxidized into H 2 O, yields a definite amount of heat, 

 1 gram of C yielding 8080 calories (see p. 584 for definition of calorie), 

 and 1 gram of H 34,460 calories; 1 gram of proteid oxidized into CO 2 

 and H 2 O yields 5778 calories; 1 gram of fat oxidized into CO 2 and H 2 O 

 yields 9312 calories; and 1 gram of carbohydrate oxidized into CO 2 and H 2 O 

 yields 4116 calories (see Potential Energy of Food, p. 302). 



Income of Heat. Since the energy-yielding food-stuffs are essentially 

 proteids, fats, and carbohydrates, and composed of C, H, O, and N, and since 

 the products of their disintegration are essentially urea, CO 2 , and H 2 O, the 

 amount of energy yielded by the oxidation of the food-stuffs can readily be 

 determined if we know the quantity and quality of the food and excreta. Since 

 the energy of the organism is manifested essentially in the form of heat and 

 work, and as under ordinary circumstances but a fraction of it is manifested as 

 work, we may in making this estimate, as a matter of convenience, consider 

 that the total available energy of the food appears in the form of heat. 



The income of energy may be estimated by determining (1) the quantity 

 of oxygen consumed ; (2) the amounts of C and H that are oxidized in the 

 body into CO 2 and H 2 O ; (3) the quantity and quality of the food, and the 

 energy yielded by the oxidation of the same substances outside the body when 

 they are decomposed into the same residual products as appear in the body ; 

 (4) the quantity of heat produced, by the aid of a calorimeter, the individual 

 being kept quiet so that as little as possible of the energy expended appears 

 as work. 



