BLOOD PRESSURE 



129 



that now the cardiac waves are decidedly the more pronounced, the respira- 

 tory, being comparatively inconspicuous. The pressure in the arteries, 

 instead of being fairly steady, undergoes very considerable alteration 

 tion during each heartbeat. 



Examination of this tracing gives us accurate information regarding 

 the blood pressure both between the heartbeats diastolic, as it is called 

 and during them systolic. It gives us a means of measuring the 

 dead load of the circulation that is, the pressure that is constantly 

 present as well as the live load that is superadded to this by each heart- 

 beat. This difference is often called the pressure pulse, and in man it 

 amounts to somewhere about 35 mm. Hg. If we take tracings with a 

 spring manometer from different parts of the arterial tree, we shall find 



160 



L ine of 

 SYSTOLIC PRESS UPE 



Of 



WEAK PRESSURE 



Line of 



DtASTOLIC 



Pressure 



PULSE 



Tfyetf/fference between 

 SYSTOLIC 



D/ASTOL/C PRESSURE 



Fig. 25. Diagram based on experiments on dogs to show the magnitude of the systolic, 

 diastolic and mean blood pressures at different parts of the circulatory system. O is the line 

 of zero pressure, and the letters below it indicate the parts of the system to which the curves 

 refer. (From Brubaker.) 



that, as we travel towards the periphery, the pressure pulse becomes less 

 and less marked, until finally by the time the capillaries are reached it 

 has almost entirely disappeared. This decline in the pressure pulse can 

 moreover be seen to be dependent more largely on a fall in systolic than 

 in diastolic pressure. In other words, the dead load of the circulation 

 the diastolic pressure remains practically constant all along the arte- 

 rial tree, whereas the systolic pressure falls relatively quickly (Fig. 25). 



Clinical Measurements 



The methods of blood-pressure measurement in man have recently 

 become so perfected that the results are almost as accurate as those ob- 



