THE COURSE OF THE BLOOD IN THE BODY 397 



fill. Almost immediately the muscles of the ventricles relax, thus 

 allowing blood to pass into the chambers within the ventricles. 

 Then, after a short pause, during which time the muscles of the 

 heart are resting, a wave of muscular contraction begins in the 

 auricles and ends in the ventricles, with a sudden strong contrac- 

 tion which forces the 

 blood out into the ar- 

 teries. Blood is kept 

 from flowing backward 

 by the valves, which 

 act in the same manner 

 as do the valves in a 

 pump. The blood is 

 thus made to pass into 

 the arteries upon the 

 contraction of the ven- 

 tricle walls. 



Practical Exercise 4. 



Why is the heart a force 

 pump? Why is the heart 

 said to be double? 



The course of the 

 blood in the body. 

 There are two distinct 

 systems of circulation 

 in the body. The 

 pulmonary circulation 



takes the blood through the right auricle and ventricle, to the 

 lungs, and passes it back to the left auricle. This is a relatively 

 short circulation, in which the blood receives oxygen in the lungs 

 and gives up carbon dioxide. The longer circulation is known as 

 the systemic circulation; in this system, the blood leaves the left 

 ventricle through the great dorsal artery called the aorta. Through 

 ever-branching arteries blood passes to the muscles, the nervous 

 system, kidneys, skin, and other organs of the body. It gives 

 up food and oxygen in these tissues, receives the waste products 

 of oxidation while passing through the microscopic tubes, capil- 

 laries, and returns to the right auricle through veins which join 



Explain how the heart is a force pump. 



