THE ARTERIAL FLOW 2 29 



the pressure in the vessels depends, but primarily by the cardiac contrac- 

 tions and the peripheral resistance. Thus, increase of blood-pressure may 

 be brought about by either, i, a more frequent or more forcible action of 

 the heart, or 2, by an increase of the peripheral resistance. On the other 

 hand, diminution of the blood-pressure may be produced, either by, a, a 

 diminished force or frequency of the contractions of the heart, or by b, a 

 diminished peripheral resistance. These different factors, however, al- 

 though varying constantly, are so combined that the general arterial pressure 

 remains fairly constant. For example, the heart may, by increased force or 

 frequency of its contractions, distinctly increase the blood pressure, but this 

 increased action is almost certainly followed by diminished peripheral re- 

 sistance, and thus the two altered conditions may balance, with the result 

 of bringing back the blood-pressure to what it was before the heart began 

 to beat more rapidly or more forcibly. 



It will be clearly seen that the circulation of the blood within the blood- 

 vessels must depend upon the diminution of the pressure from the heart 

 to the capillaries, and from the capillaries to the veins, the blood flowing in 

 the direction of least resistance. We shall presently see further that the 

 local flow also depends upon the relations between the heart's action and 

 the peripheral resistance both general and local. 



The Arterial Flow. The character of the flow of blood through the 

 arterial system depends to a very considerable extent upon the structure 

 of the arterial walls, and particularly upon the elastic tissue which is so highly 

 developed in them. 



The elastic tissue of the arteries, first of all, guards them from the sud- 

 denly exerted pressure to which they are subjected at each contraction of the 

 ventricles. In every such contraction, as is above seen, the contents of the 

 ventricles are forced into the arteries more quickly than they are discharged 

 through the capillaries. The blood, therefore, being for an instant resisted 

 in its onward course, a part of the force with which it is impelled is directed 

 against the sides of the arteries; under this force their elastic walls dilate, 

 stretching enough to receive the blood, and becoming more tense and more 

 resisting as they stretch. Thus by yielding they break the shock of the 

 force impelling the blood. On the subsidence of the pressure, should the 

 ventricles cease contracting, the arteries are able by the same elasticity to 

 resume their former caliber. 



The elastic tissue in the same way equalizes the current of blood by main- 

 taining pressure on it in the arteries during the period at which the ventri- 

 cles are at rest or are dilating. If the arteries were rigid tubes, the blood, 

 instead of flowing as it does in a constant stream, would be propelled through 

 the arterial system in a series of spurts corresponding in time to the ventric- 

 ular contractions and with intervals of almost complete rest during the in- 

 action of the ventricles. But in the actual condition of the vessels, the force 

 of the successive contractions of the ventricles is expended partly in the 



