446 



CIRCULATION OF BLOOD AND LYMPH. 



the heart beat. These arteries are, in fact, subject to very rapid and 

 very extensive changes in pressure at each beat of the heart, and 

 these changes are naturally more pronounced when the force of the 

 heart beat is increased, for instance, by muscular exercise. 



Systolic, Diastolic, and Mean Arterial Pressure. As stated 

 in the last paragraph, the arterial pressure in the larger arteries 

 undergoes extensive variations with each heart beat. The maxi- 

 mum pressure caused by the systole of the heart, the apex of the 

 pulse wave, is spoken of as systolic pressure; the minimum pressure 

 in the artery that is, the pressure at the end of the diastole of the 

 heart, or the bottom of the pulse-wave, is known as the diastolic 

 pressure. In a dog under ordinary conditions of experimentation 

 the systolic (lateral) pressure in the aorta may be as much as 168- 

 mms., while the diastolic pressure is only 100 mms. In man the 



Fig. 180. Schematic representation of the pressure change caused by each heart 

 beat. The schema represents three heart beats supposed to be recorded on a rapidly moving 

 surface by a manometer delicate enough to follow the pressure changes accurately. The 

 top of the pulse wave measures the systolic pressure; the bottom the diastolic pressure. 



systolic pressure as measured in the brachial artery may be taken 

 in round numbers as equal to 110 mms., while the diastolic pressure 

 is only 65 mms. The difference between the systolic and the 

 diastolic pressure has been designated conveniently as the pulse 

 pressure. It measures, of course, the variation in pressure in any 

 given artery caused by the heart beat, and so far as that artery is 

 concerned it gives the force of the heart beat. From the figures 

 given above it will be seen that the pulse pressure in the brachial 

 artery of man averages 45 mms. Hg. Each systole of the heart 

 distends this artery, therefore, by a sudden increase in pressure 

 equal to the weight of a column of mercury 45 mms. high. As 

 we go outward in the arterial tree the pulse pressure becomes less 

 and less, the oscillations in pressure with each heart beat are less 

 marked, until finally in the smallest arteries and capillaries and in 

 the veins there is no pulse wave, and no difference between systolic 



