288 



VETEEINAEY PHYSIOLOGY 



The suddenness of the change of pressure has a certain 

 influence on the vapidity 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 

 gentle. In the first case the change of pressure is sudden, 

 in the second case it is slow. 



Hence, if, from any cause, the pressure is raised at any 

 point, the flow will tend to be more rapid from that point 

 onwards till the normal distribution of pressure is re- 

 established. 



Friction has also a certain effect. A river runs much 

 faster in mid-stream than along the margins, because near 



AR 



FIG. 132. Diagram of the Sectional Area of the Vascular System, upon which 

 the Velocity of the Flow depends. AR, arteries ; C, capillaries ; V, 

 veins. 



the banks the flow is delayed by friction, and the more 

 broken up and subdivided is the channel, the greater is 

 the friction and the more is the stream slowed. 



When, therefore, in the capillary system the blood stream 

 is distributed through innumerable small channels, the friction 

 is very great, and this tends to dam back the blood. 



The velocity of flow in the arteries and veins may be 

 measured by various methods, of which one of the best is 

 that by means of the stromuhr, an instrument by which 

 the volume of blood passing a given point in an artery or 

 vein in a given time may be determined. 



The velocity of the flow in the capillaries may be measured 



