BLOOD PRESSURE 125 



should take. They indicate that our first consideration should be of the 

 mean blood pressure, how it is maintained, and how it can be made to 

 vary. After we have learned this, we may then proceed to a more 

 particular examination of the mechanism of the pump that is, of the 

 heartbeat ; then finally .we may proceed to examine the nature of the 

 processes by which the caliber of the arteries is controlled. 



THE MEAN ARTERIAL BLOOD PRESSURE 



The first prerequisite to the investigation of the blood pressure, as of 

 any other physical problem, is that we should possess some means by 

 which it can be quantitatively measured. The earliest attempt to accom- 

 plish this was made by the English scientist, the Rev. Stephen Hales, a 

 little over a century after Harvey published his account of the circula- 

 tion of the blood. Hales connected a glass tube nine feet in length with 

 a severed artery of a horse, the connection between the two being made 

 by means of a piece of brass pipe joined to the windpipe of a goose as a 

 substitute for rubber tubing. He found on untying the ligature on the 

 artery that the blood rose in the tube to a height of eight feet and three 

 inches above the level of the left ventricle of the heart, and that when 

 at full height it rose and fell with each pulse through a distance of two, 

 three or four inches. 



Mercury Manometer Tracings 



The somewhat crude but very significant experiment of Hales clearly 

 established the existence of the enormous pressure at which the blood is 

 made to circulate through the arteries. To render possible a further 

 investigation of the factors on which this pressure depends, it became 

 necessary to invent some more convenient means for its measurement, 

 but this was not accomplished until a century later, when Poiseuille ap- 

 plied the mercury manometer, which Ludwig subsequently adapted so 

 that tracings might be taken (Fig. 21). 



Having before us such a tracing as shown in Fig. 22, let us consider 

 how it may be used in the study of blood pressure. The first thing we 

 must do is to measure the average height of the tracing above the line of 

 zero pressure; the mean arterial blood pressure is then equal to this 

 distance multiplied by two, because the distance through which the mer- 

 cury has moved up in the limb of the manometer carrying the writ- 

 ing point is only one-half of its total displacement. Since mercury 

 is about 13.5 times heavier than an equal volume of blood, the above 

 measurement must be multiplied by this figure if we desire to express 



