PART V. 



THE PRODUCTS OF TISSUE-ACTIVITY 



CHAPTER XXII. 

 THE WASTE-PRODUCTS. 



THE CARBONACEOUS WASTE-PRODUCTS. 



The chief carbonaceous waste-product is, of course, Carbon Dioxide. 

 Only a trifling proportion of the excretion cf carbon dioxide takes 

 place through the urine, feces and sweat, the lungs playing the pre- 

 ponderating part in accomplishing the elimination of this product. 

 The total production of carbon dioxide in twenty-four hours varies 

 with the quality and quantity of food ingested, with the quantity of 

 muscular work performed, and with the rate or loss of heat from the 

 body, but in an adult male doing moderate work it may be estimated 

 in round numbers at four hundred liters at ordinary temperatures and 

 atmospheric pressure. 



The carbon-dioxide output is derived from the oxidation of the 

 carbon in the metabolized foodstuffs. It arises, therefore, in conse- 

 quence of the absorption of oxygen by the tissues. Carbon dioxide is, 

 however, not the only oxidation-product of cellular activities, and 

 hence the carbon dioxide which is given off by an animal is rarely the 

 molecular equivalent of the oxygen which is absorbed in the same 



period. The ratio: - , is termed the Respiratory Quotient 



O 2 absorbed 



and it varies in a very characteristic manner with the nature of the 

 ingested foodstuffs. Thus the carbohydrates contain a -greater pro- 

 portion of oxygen than any of the other foodstuffs, the oxygen being, 

 in fact, molecularly equivalent to the hydrogen which they contain. 

 The hydrogen in a carbohydrate may, therefore, be regarded as having 

 been completely oxidized beforehand, and the carbohydrates behave, 

 so far as the absorption of oxygen and evolution of carbon dioxide are 

 concerned, as if they consisted of pure carbon and underwent the 

 reaction : 



c + O 2 = co 2 



