112 



THE VITAL PROCESSES 



Final Disposition of Carbon Dioxide. It is readily seen 

 that the union of carbon and oxygen, which is continually 

 removing oxygen from the air and replacing it with carbon 

 dioxide, tends to make the whole atmosphere deficient in 

 the one and to have an excess of the other. This tendency 

 is counteracted through the agency of vegetation. Green 

 plants absorb the carbon dioxide from the air, decompose 

 it, build the carbon into compounds (starch, etc.) that 

 become a part of the plant, and return the free oxygen to 

 the air (Fig. 57). In doing this, they not only preserve the 

 necessary proportion of oxygen and carbon dioxide in the 

 atmosphere, but also put the carbon and oxygen in such a 

 condition that they can again unite. The force which 



enables the plant cells to decom- 

 pose the carbon dioxide is sup- 

 plied by the sunlight (Chapter 

 XII). 



Summary. Oxygen, by unit- 

 ing with materials at the cells, 

 keeps up a condition of chemical 

 activity (oxidation) in the body. 

 This supplies heat and the other 

 forms of bodily energy. Enter- 



Fiu. 57. Under surface of a i n g as a f ree element, oxygen 

 geranium leaf showing breathing ^^ ^ ^ ^ & Q the 



pores, highly magnified (O. H.). * . 



waste compounds which it helps 



to form. The free oxygen is transported from the lungs 

 to the cells by means of the hemoglobin of the red cor- 

 puscles, while the combined oxygen in carbon dioxide and 

 other compounds from the cells is carried mainly by the 

 plasma. The limited supply of free oxygen in the body 

 at any time makes necessary its continuous introduction 

 into the body. 



