THE CIRCULATION 



271 



of the stream is equal to the amount of blood passing any 

 point per second (v) divided by the sectional area of the 

 stream (S) 



where S is the radius squared multiplied by the con- 

 stant 314. 



In the vascular system the sectional area of the aorta 

 is small when compared with the sectional area of the 



AR 





FlG. 128. Diagram of the Sectional Area of the Vascular System, upon 

 which the Velocity of the Flow depends. AR., Arteries; (7., 

 Capillaries ; V., Veins. 



smaller arteries; while the sectional area of the capillary 

 system is no less than 700 times greater than that of the aorta. 

 In the venous system the sectional area steadily diminishes, 

 although it never becomes so small as in the corresponding 

 arteries, and where the great veins enter the heart it is about 

 twice the sectional area of the aorta (Fig. 128). 



This arrangement of the sectional area of the stream gives 

 rise to a rapid flow in the arteries, a somewhat slower flow in 

 the veins, and to a very slow flow in the capillaries. 



The suddenness of the change of pressure has a certain 

 influence on the rapidity of flow, as is well seen in a river. 

 If the water descends over a sudden declivity to a lower level 

 it attains a much greater velocity than if the declivity is 



