THE BLOOD PRESSURE 



987 



at the end of diastole, just before the heart begins to force a fresh 

 quantity of blood into the aorta, is the diastolic pressure ; and the 

 range between these two extremes is known as the pulse pressure. 

 Thus in the dog, with a mean pressure of about 120 mm. Hg in the 

 aorta, the systolic pressure may be as much as 160, while the 

 diastolic pressure is only 100 mm. In this case the pulse pressure 

 would be 60 mm. Hg. In man the systolic pressure, as measured 

 in the brachial artery, is under normal conditions about 110 mm., 

 while the diastolic pressure is only 65 to 75 mm., so that the pulse 

 pressure is about 45 mm. Hg. As we pass outwards towards the 

 periphery the pulse pressure becomes less and less marked, until 

 finally in the capillaries and veins there is no pulse-wave perceptible. 



THE DETERMINATION OF THE BLOOD PRESSURE IN MAN 



It is important for clinical purposes to be able to determine even approxi- 

 mately the blood-pressure in the different parts of the vascular system in man, 



and various methods have been 

 devised for this purpose. The de- 

 termination of the systolic blood 

 pressure in the arteries is easily 

 carried out by the use of Riva 

 Rocci's sphygmomanometer. This 

 apparatus (Fig. 373) consists of a 

 leather or canvas band about 10 

 cm. wide, which can be buckled 

 closely round the upper arm. In- 

 side this band is a rubber bag of 

 the same shape, which commu- 

 FIG. 373. Riva Rocci's sphygmomanometer. nicates by a rubber tube with a 

 (C. J. Martin's pattern. HAWKSLEY.) mercurial manometer and by a 



three-way tap^with a pressure 



bulb or bicycle pump, or with the external air. The band is buckled round 

 the arm and the fingers of the observer are placed on the radial pulse. 

 The bag is then distended with air so that it exercises a pressure on the arm, 

 the pressure being indicated on the mercurial manometer. Air is forced in 

 until the radial pulse disappears. By means of 

 the three-way tap the air is then slowly let out 

 of the bag until the radial pulse is just per- 

 ceptible. The height of the mercurial mano- 

 meter at this moment is equal to the systolic 

 pressure in the main arterial trunk from which 

 the brachial artery takes origin. The principle 

 of this method will be made clear by reference to 

 the diagram (Fig. 374). If we imagine A as a seg- 

 ment of the brachial artery passing through the 

 tissues which are surrounded by the rubber bag, we see that so long as the pressure in 

 the interior of the artery is greater than that on the exterior exerted by the tissues, 

 the artery will be patent and the pulse can pass through. If, however, the pressure 

 in the tissues becomes greater than the maximum pressure inside the artery at 

 any time of the heart-beat, the segment of artery will collapse (as in B), thus 

 stopping the transmission of blood and of the pulse -wave. If we exclude the 



8 



FIG. 374. 



