BLOOD PRESSURE 143 



meanwhile kept up, it will be found that the pressure between the strokes 

 is maintained at a height which is dependent on : ( 1 ) the rate at which 

 the pump is operating, and (2) the resistance to outfloAV from the tube. 

 The quicker the action of the pump and the higher the resistance, the 

 lower the fall of pressure between the beats. 



The physical explanation of this result is clearly that the fluid within 

 the elastic tube ^vhen the wave of pressure travels into it from the pump 

 distends the walls of the tube, so that when the pressure from the pump 

 ceases to act, the stretched elastic Avails recoil on the column of fluid 

 and maintain the pressure. We may say that the elastic fibers in the 

 vessel walls store up some of the systolic pressure and then transmit it to 

 the blood during diastole. 



Fig. 31. Diagram of experiment to show that the diastolic pressure depends on the elasticity 

 of the vessel wall. The pulse (produced by compressing the bulb B) disappears when fluid 

 flows through an elastic tube (F) when there is resistance (<?) to the outflow. A, basin of 

 water; B, bulb syringe; C and E, stopcocks; D, rigid tube; F, elastic tube; G, bulb filled with 

 sponge. 



These considerations would lead us to expect that patients with hard- 

 ened arteries should exhibit a lower diastolic pressure than normal per- 

 sons, which, how r ever, is not usually the case, since such patients also 

 suffer from an increase in the resistance to the flow of blood in the periph- 

 ery. The pressure pulse in these patients is, however, very marked. 

 On the other hand, when the vessel walls become more extensible and 

 elastic, as in certain cases of aneurism, the pressure pulse in the vessels 

 below the aneurism is distinctly less than that observed in normal ves- 

 sels of the same patient. 



