1 86 



ESSENTIALS OF PHYSIOLOGY. 



represents the pressure on the outer wall of the artery which exactly 

 balances the greatest pressure within the artery during systole, and at 

 which the lumen of the artery is just obliterated. 



The diastolic pressure can be approximately measured with the 

 same instrument by observing the height of the manometer when the 

 oscillations of the column of mercury with each heart beat are maximal. 

 When this happens, the pressure in the bag is just equal to that in the 

 artery at the end of diastole (diastolic pressure) ; the artery collapses 

 between the beats and then expands almost fully during systole. Thus 

 the lowest level of the m&nometer-'oetween the beats gives a record 

 of the diastolic pressure ; and by measuring the diastolic and systolic 

 pressures, the pulse pressure, which is the difference between them, can 

 be determined. The systolic pressure in the healthy adult varies in 



FIG. 60. (Starling's Principles of Physiology.} 



the large arteries, such as the brachial, from 100 to 110 mm. Hg ; it 

 becomes higher with increasing age, and at fifty, even in health, is 

 about 140 to 150 mm. Hg. It is temporarily raised during muscular or 

 mental work, and falls again during rest. 



Venous Pressure. A flat rubber bag having a hole through the 

 centre of each flat surface (fig. 60) is placed over a peripheral vein 

 and covered by a glass plate, the junctions being made air-tight with 

 glycerol ; a tube leads from the bag to a manometer and to a pressure 

 bulb. Air is blown into the bag, the glass plate being firmly held in 

 position ; when the pressure reaches a certain height the vein collapses, 

 and the reading of the manometer represents the venous pressure. 

 The same method, a smaller bag being used, may be employed to 

 determine capillary pressure. The average capillary pressure is from 

 15 to 40 mm. Hg. The venous pressure varies from 8 to 10 mm. Hg 

 in the smaller veins ; in the large veins near the heart it is only 1 or 

 2 mm. Hg, and may be negative. 



