CHAPTER VII 

 RESPIRATION 



THROUGH the movements of the blood and the lymph, 

 materials entering the body are transported to the cells, 

 and wastes formed at the cells are carried to the organs 

 which remove them from the body. We are now to con- 

 sider the passage of materials from outside the body to the 

 cells and vice versa. One substance which the body 

 constantly needs is oxygen, and one which it is constantly 

 throwing off is carbon dioxide. Both of these are constit- 

 uents of 



The Atmosphere. The atmosphere, or air, completely 

 surrounds the earth as a kind of envelope, and comes in 

 contact with everything upon its surface. It is composed 

 chiefly of oxygen and nitrogen, 1 but it also contains a 

 small per cent of other substances, such as water-vapor, 

 carbon dioxide, and argon. All of the regular constituents 

 of the atmosphere are gases, and these, as compared with 

 liquids and solids, are very light. Nevertheless the atmos- 

 phere has weight and, on this account, exerts pressure 

 upon everything on the earth. At the sea level, its 

 pressure is nearly fifteen pounds to the square inch. The 

 atmosphere forms an essential part of one's physical 

 environment and serves various purposes. The process 



1 Oxygen forms about 21 per cent of the atmosphere, nitrogen about 78 per 

 cent, carbon dioxide about .03 per cent, and the recently discovered element argon 

 about i per cent. The oxygen is in a free, or uncombined, condition the form 

 in which it can be used in the body. 



76 



