I IO 



THE VITAL PROCESSES 



to the place of low oxygen pressure at the tissues and, in so 

 doing, loads up with oxygen at one place and unloads it at 

 the other (Fig. 55). 



Passage front the Cells. Since oxygen leaves the free 

 state at the cells and becomes a part of compounds, we are 

 able to trace it from the body only by following the course 



Carbon cfioxitf* 

 press urf tot" 



Oxygen pressure 



FIG. 55. Diagram illustrating movement of oxygen and carbon 

 dioxide through the body (S. D. Magers). Each moves from a place 

 of relatively high to a place of relatively low pressure. (See text.) 



of these compounds. Three waste compounds of impor- 

 tance are formed at the cells carbon dioxide (CO 2 ), water 

 (H 2 O), and urea (N 2 H 4 CO). The first is formed by the 

 union of oxygen with carbon, the second by its union with 

 hydrogen, and the third by its union with nitrogen, hydro- 

 gen, and carbon. These compounds are carried by the 

 blood to the organs of excretion, where they are removed 

 from the body. The water leaves the body chiefly as a 

 liquid, the urea as a solid dissolved in water, and the car- 

 bon dioxide as a gas. The passage of carbon dioxide 

 through the blood requires special consideration. 



Passage of Carbon Dioxide through the Blood. Part of 

 the carbon dioxide is dissolved in the plasma of the blood, 

 and part of it is in weak chemical combination with sub- 

 stances found in the plasma and in the corpuscles. Its 

 passage through the blood is accounted for in the same 



