CELL RESPIRATION 179 



Composition of Fresh Air and of Air Expired from the Lungs 



Constituents 



In Outdoor Air 



In Air Expired 

 FROM THE Lungs 



Oxygen 



Carbon dioxide 



Nitrogen and other gases .... 

 Water vapor 



20.96 

 .04 

 79. 



variable 



16.02 

 4.38 

 79. 

 + .60 



As the table shows, there is a loss of nearly 5 per cent of oxygen, 

 and a corresponding gain in carbon dioxide and water vapor, in 

 expired air. There are 

 also some organic waste 

 substances in expired air 

 which are not shown in 

 the above table. 



Cell Respiration. — It 

 has been shown, in the 

 case of very simple ani- 

 mals, such as the amoeba, 

 that when oxidation 

 takes place in the cell, 

 energy results from this 

 oxidation. In man the 

 oxygen taken into the 

 lungs is not used there, 

 but is carried by the 

 blood to the digestive 

 tract, to the muscles, and 

 to all other parts of the 

 body where work is done. 

 Cell activity demands 

 food and oxygen. 



While work is being 

 done, certain wastes are 

 formed in the cell. Car- 

 bon dioxide is given off when carbon is burned. And when pro 

 teins are burned, other wastes containing nitrogen are formed 



Exchange of gases through the walls of an air 

 sac in the lungs. The diagram shows one air sac 

 as if laid open in two parts ; and another air sac, 

 below, intact. 



