BLOOD VESSELS 435 



If the finger be pressed upon an artery, a distinct 

 expansion, due to this rise of pressure, will be felt following 

 each systole. This is the arterial pulse. It is simply a 

 rise of pressure, and it has nothing to do with the flow of 

 blood. 



The pulse wave may be compared to a wave at sea, which 

 is also a wave of increased pressure, the only difference 

 being that, while the wave at sea travels freely over the 

 surface, the pulse wave is confined in the column of blood, 

 and manifests itself by expanding the walls of the arteries. 



If a vein be investigated in the same way, it will be 

 found that no such pulse can be detected. In the capillaries, 

 also, this pulse does not exist. 



It is best marked in the great arteries, and becomes less 

 and less distinct as the small terminal arteries are reached. 



1. Causes of the Pulse. — The arterial pulse is due to — 



1st. The intermittent inflow of blood. The arteries 

 expand from the sudden increase of pressure due to each 

 sudden rush of blood from the heart into the arterial 

 system. 



2%fZ. The resistance to outflow from the arteries into the 

 capillaries. 



If blood could flow freely from the arteries into the 

 capillaries, then the inrush of blood from the heart would 

 simply displace the same amount of blood into the 

 capillaries and the arteries would not be expanded. As 

 already indicated, the friction between the walls of the 

 innumerable small arterioles and the blood is so great that 

 the flow out of the arteries is not sufficiently free to allow 

 the blood to pass into the capillaries as rapidly as it is shot 

 into the arteries. Hence, with each beat of the heart, an 

 excess of blood must accumulate in the arteries to be passed 

 on into the capillaries and veins between the beats. 



3rd The elasticity of the walls. To allow of their 

 expanding to accommodate this excess of blood the walls 

 of the arteries must be elastic. 



It is upon these three factors that the arterial pulse 

 depends. Do away with any of them, and the pulse at 

 once disappears. 



