196 



THE CIRCULATION OF THE BLOOD 



varying constantly, are so combined that the general arterial pressure re- 

 mains 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 



FIG. 191. Schema Showing the Relation between Blood Pressure, Velocity of Flow, and 

 Vascular Area, in the Arteries, Capillaries, and Veins. Ordinates represent height of pressure and 

 speed of flow. The abscissa, b-c, represents zero pressure and speed. Space between lines a-b and 

 d-e represents arterial system ; between d-e and f-g, capillary system, and between f-g and /-*', the 

 venous system. Line A-B equals pressure; line C-D, speed of flow; and line E-F, vascular area. 

 (Modified from Gad.) 



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, 



