THE VASCULAR MECHANISM. 



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a uniform pressure throughout the vascular system, the height of which 

 will depend upon the amount of blood present in the vessels. This 

 is called the mean systemic pressure. If more blood is added to the 

 vascular system, the mean pressure will rise, and vice versa. When 

 the heart starts to beat, it pumps blood into the arteries, which 

 become distended, because at first the outflow from the arterioles is 

 less than the amount of blood forced into the aorta ; and the arterial 

 pressure rises. As a result of the transference of blood from the veins 

 to the arteries the venous pressure falls below the mean pressure. It 

 follows, first, that under normal conditions the arterial and venous 

 pressures vary inversely with one another, and, secondly, that the 

 arterial, capillary, and venous pressures will all vary with the total 

 volume of blood in the vascular system, although the venous arid 

 capillary pressures are those chiefly influenced. 



BLOOD PRESSURE IN MAN. 



Arterial Pressure. The highest blood pressure occurring during 

 the cardiac systole is called the systolic pressure ; the pressure corres- 

 ponding with the end of diastole is the diastolic pressure, the difference 

 between the two being called 

 the pulse pressure. The sys- 

 tolic pressure is measured by 

 means of a Riva-Rocci sphyg- 

 momanometer. This consists 

 (fig. 59) of a leather band 

 about four inches wide, inside 

 which is a rubber bag com- 

 municating with a mercurial 

 manometer and connected 

 with a small pressure bulb. 

 Attached to the bulb is a 

 screw by which the bag can be put into communication with or shut 

 off from the external air. 



The band is fastened round the upper arm. The observer feels the 

 radial pulse with the fingers of one hand, while with the other he 

 squeezes the bulb and distends the bag with air, until the pressure is 

 just sufficient to obliterate the brachial artery and the radial pulse 

 disappears. When this occurs, the pressure in the mercurial manometer 

 is noted. The screw attached to the bulb is then gently turned, the 

 air slowly escapes, and the pressure falls ; when the radial pulse is just 

 perceptible, the pressure in the manometer is again observed. The 

 mean between the two readings is the systolic arterial pressure. This 



FIG. 59. Riva-Rocci sphygmomanometer. 

 (From Messrs Hawksley.) 



