272 HUMAN PHYSIOLOGY 



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 

 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 in transparent 

 structures by means of a microscope with an eye-piece 

 micrometer. (Practical Exercise.} The velocity of the 

 blood is 



Carotid of the dog about . . 300 mm. per sec. 

 Capillaries about . . . 0-5 to 1 m. 

 Vein (jugular) about . .150 mm. 



It is not so easy to give definite figures for the velocity of 

 the lymph stream. 



Disturbance of any of the factors which govern the rate 

 of flow will bring about alterations in the velocity of the 

 blood in arteries, capillaries, and veins. Thus, an increased 

 venous pressure, by leading to a diminution in the difference 

 of pressure between arteries and veins, will materially slow the 

 blood stream. Great dilatation of the arterioles will slow the 

 blood stream in them ; and increased viscosity of the blood 

 by increasing friction with the vessel wall will also slow the 

 stream. 



