CHAPTER -XVII 



THE REMOVAL OF THE END-PRODUCTS OF 

 METABOLISM 



THE statement has repeatedly been made in varying 

 form that the bulk of the food is taken for the sake of its 

 potential energy. Either at once or after storage it is 

 oxidized, and the energy turned to account for tempera- 

 ture maintenance and for the performance of muscular 

 work. The main products are carbon dioxid and water. 

 These are likewise the chief products formed when familiar 

 fuels are burned outside the body. Wood, coal, and gas 

 yield the two in great quantity and only small amounts of 

 other compounds. Hence the primary problems of excre- 

 tion concern the manner of elimination of carbon dioxid 

 and water. 



The water leaving the body during twenty-four hours 

 may be 2 or 3 kilograms. The carbon dioxid discharged 

 in the same period is not often in excess of 1 kilogram. 

 Nevertheless, we say that carbon dioxid is the leading 

 waste-product of animal life. This is justified by the con- 

 sideration that by far the larger part of the water which we 

 measure is merely water previously received in the same 

 state. To this large volume the tissues have added a mod- 

 erate quantity of water say 250 grams which is a true 

 metabolic product. This has been formed by the oxidation 

 of compounds containing hydrogen. The water output 

 of the body is inevitably greater in the long run than the 

 water income. This fact may be disguised on single days 

 by water retention. 



Carbon Dioxid Elimination. Respiration has been 

 defined as the process within the living cells in course of 

 which complex organic molecules are decomposed and 



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