112 THE HEART AND CIRCULATION. 



aorta, and in 22 or 23 beats all the blood in the body 

 passes through the heart. The power exerted by the 

 heart every minute in thus driving the blood upon its 

 course has been estimated as sufficient to raise its own 

 weight, three-quarters of a pound, the height of the 

 Washington monument or 150 meters; for the ventricles 

 have to force the blood into vessels already full. 



Factors Affecting Circulation. There are three main 

 factors in the circulation: 1. the systole, which gives the 

 blood its first impulse; 2. the peripheral resistance in the 

 capillaries, which serves to hold it in check, slowing the 

 circulation and doing away with its rhythmic character, 

 and 3. the elasticity of the walls of the arteries. 



If a ligature is tied about an artery, there is a swelling 

 on the side toward the heart, while in the case of a vein, 

 the swelling is on the side away from the heart, that is, 

 the swelling is in either case on the side from which the 

 blood comes. When an artery is cut, however, the blood 

 comes out rhythmically in spurts, though from a cut vein 

 it oozes slowly and regularly. For the blood is pumped 

 out by the heart rhythmically and its rhythmic beating 

 against the walls of the artery is felt in the pulse, which 

 follows slightly after the beat of the heart itself. The 

 pulse is due to the fact that the vessels into which the 

 blood is forced are already full. This causes a local 

 dilation at the beginning of the artery which passes with 

 diminishing force along its entire length, the distention 

 being due to the fact that more force is needed to drive 

 the blood through the small arteries and capillaries 

 than to stretch the elastic walls of the aorta and the large 

 arteries. It is this elastic character of the arteries that 

 makes the blood flow constant, for otherwise the blood 

 would come intermittently in jets, as it is pumped from 

 the heart. The elastic walls of the vessels, however, 

 offer a certain resistance to the pumping of the fluid 

 through them and at the same time, by relaxing between 

 whiles, allow a certain amount of fluid to be retained in 

 them, so that they continue full and the flow is more or 



