VELOCITY AND PRESSURE OF BLOOD-FLOW. 495 



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, 1 y 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. 194. 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 t9 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 to 116 mms., while the diastolic 

 pressure is only 65 to 75 mms. The difference between the 

 systolic and the diastolic pressure has been designated con- 

 veniently 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 except for the small component used to accelerate 

 the movement of the blood. 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 



