340 



Our Surroundings 



carried off by the blood, the liquids being carried to the kidneys 

 to be excreted, and the carbon dioxide to the lungs where it is 

 exhaled. 



The oxidation of digested food substances in the cells may be 

 compared to the burning, or oxidation, of wood in a stove. The 

 oxygen, coming into the stove in the draft of air, unites chemically 

 with the carbon and the hydrogen in the wood, producing heat 

 energy, carbon dioxide, and a little water vapor. Both the wood 

 and the oxygen change their forms. Part of the oxygen 

 unites with the carbon, forming carbon dioxide, and part unites 

 with the hydrogen, forming water vapor. The carbon dioxide and 

 the water vapor pass up the chimney with the smoke, and a 

 mineral part of the wood, called ashes, remains. During this 

 process heat is set free. Thus oxidation does not actually destroy 

 the material burned but causes it to assume different forms. 



In the body, the draft is represented by the air taken into the 

 lungs. From this air, the oxygen necessary for burning is ab- 

 sorbed. The fuel is represented by digested food substances 



absorbed and distributed by 

 the blood to all cells. In body 

 oxidation, there is no flame, 

 but energy is given off. This 

 energy supplies heat, as does 

 the fire, and it makes possible 

 our motion and other activities. 

 The ashes of the fire are rep- 

 resented by the waste products 

 of oxidation in the body which 

 are carried off by the blood. 

 As in the fire, then, fuel and 

 oxygen are necessary to com- 

 bustion, and combustion re- 

 sults in changes in matter. 



The Respiratory System. 

 The most important organs 

 of respiration are the diaphragm, the ribs, the lungs, the red cor- 

 puscles of the blood, and the cells of the body. These, together 



