I0 8 THE VITAL PROCESSES 



which are to become a part of the cells. Both of these 

 uses, however, are of minor importance when compared 

 with the main use of oxygen, which is that of an aid in sup- 

 plying energy to the body. 



Oxygen and the Maintenance of Life. In the supplying 

 of energy to the body, one of the conditions necessary to 

 the maintenance of life is provided. Because oxygen is 

 necessary to this process, and because death quickly results 

 when the supply of it is cut off, oxygen is frequently called 

 the supporter of life. This idea is misleading, for oxygen 

 has no more to do with the maintenance of life than have 

 the food materials with which it unites. Life appears to 

 be more dependent upon oxygen than upon food, simply 

 because the supply of it in the body at any time is exceed- 

 ingly small. Being continually surrounded by an atmos- 

 phere containing free oxygen, the body depends upon this 

 as a constant source of supply, and does not store it up. 

 Food, on the other hand, is taken in excess of the body's 

 needs and stored in the various tissues, the supply being 

 sufficient to last for several days. When the supply of 

 either oxygen or food is exhausted in the body, life must 

 cease. 



The Oxygen Movement a Necessity. Since free oxygen 

 is required for keeping up the chemical changes in the 

 cells, and since it ceases to be free as soon as it goes into 

 combination, its continuous movement through the body is 

 a necessity. The oxygen compounds must be removed as 

 fast as formed in order to make room for more free oxygen. 

 This movement has already been studied in connection 

 with the blood and the organs of respiration, but the con- 

 sideration of certain details has been deferred till now. 

 By what means and in what form is the oxygen passed to 

 and from the cells? 



