324 THE ANIMALS AND MAN 



This completes the systemic circulation. 



Pulmonary circulation. — We have learned that the sys- 

 temic arteries carry blood away from the left ventricle, and 

 that the veins return it to the right auricle. In this circula- 

 tion the blood gathers food by absorption from the intestines, 

 gives up food and oxygen to all the tissues and gets rid 

 of certain waste products in the kidneys and liver, but it 

 must also get rid of the large amount of carbon dioxide 

 collected from the tissue cells. It does this through the 

 pulmonary circulation. 



The blood received into the right auricle from the system 

 (through the vena cavas) is forced into the right ventricle 

 through the tricuspid valve. It is then pumped out into the 

 pulmonary artery which carries it to the lungs, as already 

 stated. The pulmonary arteries break up into capillaries in 

 the tissues of the air sacs of the lungs (fig. 176). Carbon 

 dioxide in the blood is here exchanged for oxygen. The 

 blood thus purified is collected by four large pulmonary 

 veins (fig. 175, 13, 14, 15, 16) and sent back to the left 

 auricle of the heart. Figure 165 shows the course of this 

 blood-flow as just described. 



We see, therefore, that all the blood leaves the heart by 

 way of arteries and returns to the heart by way of capillaries 

 and veins. We see that it all passes through one set of cap- 

 illaries and that the blood from some of the organs (stomach, 

 intestines, spleen and pancreas) passes through a second set 

 of capillaries in the liver before returning to the heart. 



How is the circulation of blood maintained? — The 

 circulation of blood is effected as follows: The muscular 

 ventricles pump the blood into the arteries. The valves be- 

 tween the ventricles and arteries prevent the reflux of blood. 

 The fine capillaries, because of their immense number and 

 small size, exert a certain resistance so that all the blood thus 

 pumped out of the heart can not at once pass through them. 

 The walls of the arteries are elastic and hence expand to ac- 



