THE BLOOD PRESSURE 



991 



produced either by raising the level of a or by lowering the level of 

 the reservoir and so the pressure at A. 



The difference of pressure between any two points, i.e. between 

 D and E, may be regarded as that pressure which is necessary to main- 

 tain a certain velocity of the fluid against the resistance offered by the 

 friction of the fluid in contact with the walls of the tube. This friction, 

 and therefore the resistance to the flow, can be altered by diminishing 

 the diameter of the tube, when a larger difference of pressure will be 

 necessary in order to maintain the same velocity of flow. This can be 

 shown by introducing a resistance between D and E by partially clamp- 



H 



ing the tube at this point (Fig. 379). The continuity of the fall of pres- 

 sures in the vertical tube is at once abolished. Between A and D there is 

 a continuous fall, which is succeeded by a steep fall between D and E, 

 and this again by a gradual fall between E and G. In any system of 

 tubes therefore through which fluid is flowing the fall of pressure 

 between any two points will be proportional to the velocity of the flow 

 between these two points. The velocity, on the other hand, will vary 

 directly as the difference of pressures, and inversely as the resistance 

 between the two points, which may be expressed by the formula 



In the vascular system, while the circulation is maintained, the 

 largest difference of pressure exists between the arteries on ,the one 

 side and the small veins on the other, a great fall occurring (between the 

 arteries and the capillaries themselves. This distribution^ pressure 

 points to the chief resistance in the vascular system as being situated in 

 the arterioles. The resistance presented by these vessels is due to the 

 fact that they are maintained in a state of tonic contraction . by, 

 the agency of the central nervous system. The total bed of the 



