MECHANICS OF THE CIRCULATION IN THE VESSELS 



Measurement of the Blood-Pressure in Man. In man the blood- 

 pressure has been estimated by adjusting over an artery an instru- 

 ment known as a sphygmomanometer or sphygmometer, which, in 

 its most modern form, consists essentially of a hollow rubber pad or 

 bag containing air, and connected with a metallic pressure gauge or 

 a mercurial manometer. 



The simplest method is that devised by Riva-Rocci (Fig. 44). An 

 armlet in the form of a broad rubber bag, supported externally by 

 canvas or leather, is adjusted round the upper arm. The interior of the 

 bag is connected with a mercury manometer, and also with a strong 

 rubber bulb provided with a valve. By rhythmical compression of 

 the bulb the pressure can be raised. Between the pressure bulb and 

 the rest of the system is a thin rubber balloon, which by its distension 

 renders the changes of pressure more gradual. The finger of the 

 observer is placed over the radial artery, ^nfl th* 

 pressure is raised until the, pulse disappears. Then 

 the pressure is allowed to fall gradual! vT~ and the 

 manometer reading at the moment when the pulse 

 first reappears in the radial gives the maximum or 

 systolic pressure in the brachial artery. 



Instead of palpating the radial artery, a stetho- 

 scope may be placed over the brachial just below 

 the edge of the armlet, according to the method of 

 Korotkoff , by which, in addition to the systolic, the 



determined^ This is much the best of 

 all the methods of measuring the arterial 

 pressures in man. The pressure is 

 raised somewhat above that necessary 

 to obliterate the pulse, and then allowed 

 to fall slowly. At the moment when 

 pulsations first begin to break through 

 below the armlet, a succession of sharp 

 taps, synchronous with the pulse, is 

 heard. The tapping sound grows rapidly 

 louder as the artery opens up more and 

 more, then abruptly diminishes and 

 changes its character, and gradually dis- 

 appears. Several phases have been distinguished after the first maxi- 

 mum. Everybody agrees that the pressure shown by the manometer 

 thft gmmd is first heard is the svst<">1i>. jr^gg^r^ This corresponds 



Fig. 44. Riva-Rocci Apparatus. 

 a, armlet; b, manometer tube; 

 c, bottle containing mercury, 

 into which b dips ; d, thin 

 rubber bulb ; e, thick rubber 

 bulb for getting up pressure. 



, 



closely with the systolic pressure as determined by palpating the radial ; 

 and it can be shown experimentally that at pressures in the armlet 

 exceeding this the lumen of the brachial artery is actually obliterated 

 and not merely narrowed to such a degree as to prevent the passage of 

 the pulse wave, while still permitting the passage of some blood (see 

 Practical Exercises, p. 213). 



The diastolic pressure is the, pressure at which a. sudden ^weakening 

 arm-firming ol the sound occurs (beginning of the fourth phase), and not 

 tne pressure at which the sound becomes altogether inaudible (Mac- 

 William, etc.). The sound seems to be due to vibrations set up in the 

 walls of the artery and the structures in con-tact with it, when it is 

 suddenly opened by the pulse waves. According to Erlanger, the essen- 

 tial thing is the ' water hammer ' action of the blood when, moving 

 through the artery under compression in the armlet, it strikes the stag- 

 nant blood in the uncompressed artery below and distends it. 



