BLOOD VESSELS 465 



rate of flow must be decreased. This occurs in various 

 forms of heart failure, and when the heart has an insufficient 

 supply of blood to contract upon. 



1. Velocity. 



The velocity of the flow of a fluid depends upon the 

 width of the channel. Since in unit of time unit of volume 

 must pass each point in a stream, if the fluid is not to 

 accumulate at one point, the velocity must vary with the 

 sectional area of the channel. In other words, the velocity 

 (V) 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) — 



V 



where S is the radius squared multiplied by the constant 



3-14. 



In the vascular system the sectional area of the aorta is 

 small when compared with the sectional area of the smaller 

 arteries ; Avhile the sectional area of the cajnllary system may 

 be 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. 195). 



This arrangement of the sectional area of the vascular 

 system gives rise to a rapid flow in the arteries, a somewhat 

 slower flow in the veins, and 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 from any cause the pressure is raised at one point, 

 the flow will tend to be more rapid from that point 

 onwards till the normal distribution of pressure is re- 

 established. When the difference between the pressure 

 at the arterial end and at the venous end of a set of 

 capillaries is increased, a more rapid flow of blood takes 

 place through the tissues. 

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