BLOOD PRESSURE 



127 



The tracings taken with such a manometer, as shown in Fig. 24, are 

 quite different from those with the mercury manometer. It will be seen 

 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 

 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- 



120 



L ine of 

 SYSTOLIC PRESS UPE 



Line of 

 MEAN PRESSURE 



Line of 



D/ASTOLIC 



Pressure 



SYSTOLIC 



arrc/ 

 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.) 



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 

 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). 



*The tracings shown in Figs. 22 and 24 are not typical, the pulse pressure being too small in 

 the latter and too large in the former. 



