BLOOD PRESSURE. 289 



impediment offered to the escape of the blood from the arteries 

 that thus keeps them distended ? If the arteries and veins were 

 a set of continuous wide tubes of similar construction and capacity 

 throughout, it would be impossible for the heart to empty the 

 veins, to overfill the arteries, and to establish the great pressure 

 difference that normally exists. Therefore some resistance equal 

 to the pressure must be offered to the flow of the blood from the 

 arteries into the veins. 



This resistance is made up of several items, of which one alone 

 is sufficient to keep up the arterial pressure, namely, the active 

 contraction of the arterioles. No doubt the enormous increase of 



FIG. 130. 



Tracing, showing the effect of Stimulation of Vagus Nerve. Stimulus 

 applied between vertical lines. (Recording surface moved from left to 

 right.) 



surface over which the blood has to move in the capillaries, and 

 the pressure exercised upon them by the surrounding elastic tis- 

 sues, impede the emptying of the arteries. But that the contrac- 

 tility of the arterioles is the most important item, may be seen 

 from the following consideration. The resistance offered by the 

 capillaries themselves, whether by the greater friction or by the 

 elastic pressure, is insignificant when compared with the arterial 

 blood pressure, and is far from sufficient to restrain the blood 

 from rushing into the veins. This is seen when the arterioles are 

 paralyzed by the destruction of the nervous mechanism controlling 



