CHAPTER XXXVI. 



THE CHEMICAL AND PHYSICAL CHANGES IN THE 

 AIR AND THE BLOOD CAUSED BY RESPIRATION. 



The Inspired and the Expired Air. The inspired air, atmos- 

 pheric air, varies in composition in different places. The essential 

 constituents from a physiological standpoint are the oxygen, 

 nitrogen, and carbon dioxid. The new elements argon, krypton, 

 etc. have not been shown to have any physiological significance, 

 and are included with the nitrogen. The accidental constituents 

 of the air vary with the locality. In average figures, the composi- 

 tion of this air is, in volume per cent.: nitrogen, 79; oxygen, 20.96; 

 carbon dioxid, 0.04. The expired air varies in composition with 

 the depth of the expiration and, of course, with the composition of 

 the air inspired. Under normal conditions the expired air contains, 

 in volume per cent. : nitrogen, 79; oxygen, 16.02; carbon dioxid, 

 4.38. In passing once into the lungs the air, therefore, gains 4.34 

 volumes of carbon dioxid to each hundred, and loses 4.94 volumes 

 of oxygen. 



N. o. co,. 



Inspired 79 20.96 0.04 



Expired 79 16.02 4.38 



4.94 4.34 



This table expresses the main fact of external respiration: the 

 respired air loses oxygen and gains carbon dioxid and consequently 

 the blood absorbs oxygen and eliminates carbon dioxid. It will be 

 noted, also, that the volume of oxygen absorbed is greater than the 

 volume of carbon dioxid given off. This discrepancy is explained 

 by the general fact that the oxygen absorbed is used in the long run 

 to oxidize the carbon and also the hydrogen of the food; conse- 

 quently, while most of it is eliminated in the expired air as carbon 

 dioxid, some of it is excreted as water. For the sake of complete- 

 ness it may be stated that traces of hydrogen and methane are also 

 found in the expired air. They probably originate in the intestines 

 from fermentation processes and are carried off in solution in the 

 blood. 



665 



