CHAP. X CIRCULATION OF THE BLOOD 131 



properly returned into the blood, poisonous waste mate- 

 rials are at once removed and so cannot interfere with 

 the free and natural action of the cells. Exercise opens 

 all the drainage tubes and renovates the entire organism. 



98. Course of the Blood. We saw that the as ce tid- 

 ing vena cava returns the blood from the lower extrem- 

 ities, the pelvic and abdominal organs, in fact, from the 

 entire body below the level of the heart ; and the de- 

 scending vena cava returns the blood from the head, 

 arms, and chest, or from the body above the level of 

 the heart. These two trunks empty into the right 

 auricle. As soon as the right auricle is full its muscular 

 walls begin to contract. The blood cannot return into 

 the large veins but must flow into the right ventricle. 

 When that chamber is filled it begins to contract. Then 

 the tricuspid valve closes and prevents the blood from 

 returning into the auricle. The only opening left is the 

 one into the pulmonary arteries which carry the blood 

 to the lungs. 



You remember how the dark blood that comes to the 

 capillaries of the lungs is exposed to the oxygen of the 

 air sacs. Here the blood loses carbon dioxid, organic 

 impurities, etc., and gains a very large amount of 

 oxygen. In this way the blood is said to be purified. 

 The dark red, almost black, blood becomes bright red 

 or scarlet. It is now sent back through the pulmonary 

 veins to the left auricle. 



The blood passes the mitral valve and goes into the 

 left ventricle. When that chamber is full, the blood is 

 forced out into the aorta. The mitral valve closing pre- 



