XIII. CIRCULATION ^continued). 

 The Flow or Blood, and Blood Systems. 



In the preceding chapter we have seen the way in which 

 the various parts of the circulatory system are put together. 

 In the present chapter we shall investigate the cause of 

 the blood flow and its course in the blood vessels. 



Heart action. — (See Fig. 66.) Enough of the structure 

 of the heart has been outlined to show that the actual 

 driving of the blood into the arteries is due to the con- 

 tractions of the ventricles. Before these ventricles contract 

 it is necessary that they be filled with blood, and this 

 requires a preliminary contraction on the part of the auri- 

 cles. The- order of these contractions is as follows: The 

 action begins at the mouth of the large veins (the vense 

 cavffi). The blood is flowing steadily from these veins into 

 the auricles and dilating them. Next the tricuspid and 

 the mitral valves are forced open by the blood pressure 

 in the auricles and the ventricles fill. Now the auricles 

 contract and send a wave into the ventricles dilating them 

 still further and checking the flow from the veins by back 

 pressure, while the mitral and tricuspid flaps float upward. 

 As the ventricle becomes fully filled, it, in turn, begins to 

 contract, and these flaps are suddenly pressed upward 

 until they cover the opening tightly and prevent further 

 flow from the auricle into the ventricle and also back 

 flow from the ventricle into the auricle. The chordae 

 tendinse prevent the flaps from being forced back through 

 the opening. The blood in the ventricles is now under 



191 



