VELOCITY AND PRESSURE OF BLOOD-FLOW. 



445 



doubtedly indicates their extent with entire accuracy. But when 

 these changes are very rapid, as in the beat of a dog's or rabbit's 

 heart, the mercury does not register either extreme in the variation, 

 but tends to record the mean or average pressure. The full extent 

 of the variations in arterial pressure caused by the heart beat can be 



Fig. 179. Typical blood pressure record with mercury manometer: Bp, The record 

 showing the heart beats and the larger curves due to the respirations (respiratory waves 

 of blood-pressure) and still longer waves due to vasomotor changes; T, the time line, giving 

 the time in seconds. The actual arterial pressure at any moment is the distance from the 

 base line that is, the line of zero pressure to the blood-pressure line, multiplied by two. 

 These values are indicated in the vertical line drawn to the right, which shows that the 

 average pressure at the time of the experiment was 100 mms. Hg. The small size of the 

 variations in pressure due to each heart beat is altogether a false picture due to the inertia 

 of the mercury, its inability to follow completely the quick change. Each heart beat, 

 instead of being smaller, should be larger than the respiratory waves. 



determined by other means (see below), and, if the knowledge thus 

 obtained is applied to the correction of the record of the mercury 

 manometer, the tracing given in Fig. 179 should have, so far as the 

 heart beats are concerned, somewhat the appearance shown in Fig. 

 180. This latter figure gives a more accurate mental picture of 

 the actual conditions of pressure in the large arteries, as influenced by 



