THE R ESP IRA TOR Y S YSTEM 1 49 



a mixture of oxygen and nitrogen in the proportion of 

 about 21 parts of oxygen to 79 parts of nitrogen. It 

 also contains about .05 per cent, of carbon dioxid, a 

 variable quantity of moisture, and some traces of am- 

 monia. 



If we examine the air at expiration, after passing through 

 the lungs, we shall find that it has become altered in the 

 following particulars : First, it has lost part of its oxygen ; 

 second, it has gained carbon dioxid, moisture, and or- 

 ganic matter. 



The amount of carbon dioxid given off in the expired 

 air, or the amount of oxygen that it contains, is dependent 

 upon the gross amount of carbohydrates that have been 

 consumed by the individual. Tigerstedt, in his descrip- 

 tion of this important action, says: "The percentage of 

 oxygen in the expired air is, of course, less than that of the 

 inspired air, and, in fact, it decreases more, as a rule, than 

 the per cent, of CO 2 (carbon dioxid) increases." When 

 carbon burns in oxygen, the volume of the gas does not 

 change. Since in respiration, however, the amount of 

 oxygen which has disappeared is greater than that of 

 the carbon dioxid formed, it follows that the oxygen is 

 used in the body for other oxidations than that of carbon. 

 The ratio between carbon dioxid formed and oxygen used, 



CO, . 

 .- , is called a respiratory quotient. 



U 2 



The value of the respiratory quotient is very different 

 under different circumstances, and depends upon the kind 

 of food-stuffs which at the time are being burned in the 

 body. The carbohydrates contain in their molecule 

 just as much oxygen as is necessary to completely utilize 

 their hydrogen. The total quantity of the inspired oxygen, 

 therefore, can be used for oxidation of their carbon. 

 The nurse's attention is directed to the following classi- 

 fication of the volumes of air at rest and in motion during 



