CHAPTER XX. 

 THE GASEOUS EXCRETION. RESPIRATION. 



In the last chapter the amount of nitrogen excreted with the 

 urine was discussed at some length. With the nitrogen certain 

 corresponding proportions of carbon, hydrogen and oxygen are 

 excreted in the urea, uric acid and other bodies described. But 

 the larger amounts of these elements are thrown off from the 

 body in different form, and especially in the carbon dioxide 

 and water vapor eliminated in respiration and perspiration. 

 From certain classes of foods the end products formed are 

 these two only when the oxidation is ideally complete. This 

 is the case with the fats and carbohydrates, and supposing 

 them wholly burned in the body the final results are repre- 

 sented in this way, taking typical substances for illustration : 



CeH 12 6 + 60 2 = 6C0 2 + 6H 2 0, 

 C3H B (Ci 8 H 35 O 2 ) 3 + 1630 = 57CO 2 + 55H 2 O. 



In the actual behavior of these compounds in the human 

 body, however, the results are somewhat different. The oxi- 

 dation is never quite as complete as here indicated, as traces of 

 both carbohydrates and fats are left in more complex forms. 



THE RESPIRATORY QUOTIENT. 



In studying the completeness of oxidation of certain foods 

 much has been learned by a consideration of the factor known 

 as the respiratory quotient which is simply the ratio of the 

 carbon dioxide eliminated to the oxygen absorbed, measured by 

 volume. This quotient is therefore given by the expression 

 CO 2 /O 2 . For the sugar of the above equation we require six 

 molecules of oxygen, and the carbon dioxide produced is also 

 six molecules. Hence CO 2 /O 2 = i. For all common car- 



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