24 



GASEOUS METABOLISM OF INFANTS. 



represent the average of a large number of analyses, and have been exten- 

 sively used in computations of the indirect calorimetry of men. 



Table 12. — Chemical composition of the constituents of the body. 1 



1 While these values were determined on carefully isolated and purified materials obtained 

 from the animal body, they may be considered as approximate values for all proteins and fats. 



Innumerable analyses have been made of the ordinary food sub- 

 stances, but the composition of starch, cane sugar, glucose, and lactose 

 can be computed from the chemical formulas directly. For the com- 

 position of normal fat, the average values given by Koenig are ordi- 

 narily used. 1 When these substances are burned inside the body, a 

 definite volume of oxygen combines with their carbon and hydrogen to 

 produce carbon dioxide and water. The amount of carbon dioxide 

 produced per gram of a substance, the amount of oxygen required for 

 the oxidation, and the total heat evolved can be determined exactly by 

 burning a known amount of various fats and carbohydrates outside of 

 the body, as, for instance, in a calorimetric bomb. 



The carbon dioxide is excreted in a gaseous form, while the water 

 may be excreted through the kidneys, vaporized through the lungs and 

 skin, or added to the residual water always present in the body. Since 

 there is so large a storage of water in the body, it is obviously impossible 

 to distinguish between water formed by the oxidation of organic mate- 

 rial and water existing preformed in the body, but the relationship 

 between the oxygen consumed and the carbon dioxide produced has a 

 great physiological value and plays an important role in indicating 

 the character of the material burned in the body. The importance of 

 this relationship was early recognized by Pfliiger and the ratio was 

 designated by him as the "respiratory quotient." 



The theoretical respiratory quotient for the combustion of a pure 

 substance of definite chemical composition may be easily computed. 

 If we consider, for example, one of the chief foods of an infant — lactose 

 or milk sugar, with a chemical formula of C12H22O11 + H 2 — we see 

 that the hydrogen and oxygen are present in the molecule in the exact 

 proportions to form water. Since there is in the molecule sufficient 

 oxygen to oxidize the hydrogen completely, the oxygen which enters 

 into the combustion burns only the carbon. It is, of course, obvious 

 that the complete combustion does not proceed in this sharply defined 

 manner, but this alters in no wise the trend of our reasoning. 



koenig, Chemie der menschlichen Nahrungs- und Genussmittel, 3d ed., 1 , p. 198. 



