CHAPTER XIII 

 THE CIRCULATION OF THE BLOOD 



System of tubes for the blood. In order that the 

 blood may surely find its way from tissue to tissue in 

 the bodj^ a system of tubes and special organs has been 

 developed. This system has a central pumping station, 

 the heart, which furnishes the power by which the blood 

 is pushed forward. A system of tubes, the arteries, veins 

 and capillaries, leads from the heart to the tissues and 

 back to the heart again. These tubes are so constructed 

 and arranged that, although the blood is carried to all 

 parts of the body through their many branches, yet it is 

 all gathered together again and returned to the heart. 



Greater and lesser circuits. The blood leaves the 

 large cavity on the left side of the heart and goes to 

 the tissues through thick walled tubes, the arteries. It 

 returns by larger and thinner walled tubes, the veins, 

 to the right side of the heart. It is then sent through 

 another set of arteries to the lungs, whence it again 

 returns to the left side of the heart through a second set 

 of veins, and is once again ready for a repetition of its 

 journey to the tissues. There is thus the main circu- 

 latory route to and from the tissues comprising the 

 greater, or systemic, circuit ; and a secondary route to 

 the lungs and return, which is known as the lesser, or 

 pulmonary, circuit. Neither of these circuits is in itself 

 complete, since the blood after returning from the tissues 

 can reach the starting point in the heart for its next 

 trip to the tissues only by way of the pulmonary route. 



135 



