RESPIRATION 187 



the air of the lungs and those in the blood; and in the 

 same way the oxygen passes from the blood to the tis- 

 sues, while the carbonic acid passes from the tissues into 

 the blood. 



266. Atmospheric Air compared with Expired Air. The 

 atmosphere is composed of five gases : oxygen, nitrogen, 

 argon, carbon dioxide, and watery vapor, the last varying 

 greatly in relative amount according as the atmosphere is 

 moist or dry. Helium and other recently discovered 

 gases exist in the air in very small traces, but may be left 

 out of consideration. Oxygen and nitrogen are the prin- 

 cipal elements in air. The amount of carbon dioxide in 

 outdoor air is very small. 



Pure dry air contains in each one hundred parts 20.96 

 parts of oxygen, 78 parts of nitrogen, 1 part of argon, and 

 .04 part of carbon dioxide. Expired air (breathed once) 

 contains about 16 parts by volume of oxygen, 79 parts 

 nitrogen and argon, and 4 parts carbon dioxide. There is, 

 besides, in expired air, a considerable quantity of watery 

 vapor, and a variable amount of volatile organic matter, 

 not sufficient generally for chemical analysis to detect, but 

 often perceived in a close room by the sense of smell. 



It appears from the above that more than four parts of 

 the oxygen is appropriated from the air in the lungs, and 

 that about four parts of carbon dioxide, besides a minute 

 amount of organic waste and some vapor of water, are 

 given back to the air. The temperature of the expired 

 air is very nearly the same as that of the body. 



267. Effect of Respiration on the Blood. The blood 

 brought to the lungs by the pulmonary artery is dark 

 purple in color, but that which is returned to the left 

 auricle of the heart, after circulating through the lungs, 

 is of a brilliant scarlet. The same difference is seen 



