32 PHYSIOLOGY FOR NURSES 



As ordinarily understood blood pressure means arterial 

 pressure and is taken during systole, systolic pressure 

 or, during diastole, diastolic pressure. The method of 

 determining pressure is to encircle the arm (because 

 there is but one bone and the muscles, when compressed, 

 squeeze the arteries uniformly) with a long rubber sac, 

 enclosed in leather, which is connected by tubing Avith 

 a column of mercury and an air pump. "When air is 

 pumped into the sac until the blood can no longer pass 

 under it, as shown by disappearance of the radial pulse, 

 the height of the column of mercury is read. This is 

 systolic pressure and, in healthy young men, varies be- 

 tween 110 and 130 mm. of mercury. 



Pressure Forces. Pressure forces are three in num- 

 ber: i.e., the power of the heart, the resistance ot the 

 vessels, and the quantity of blood. In normal animals, 

 the two former change with changing conditions, so 

 that a practically constant level of pressure is main- 

 tained the greater part of the time. Decrease in the vol- 

 ume of the blood tends to lower the pressure, but, within 

 certain limits, this is compensated for by increase in the 

 rate of the heart and a constriction of the vessels. Like- 

 wise, if fluid is introduced into the vessels, it does not 

 cause a marked and persistent rise in the pressure, due 

 to the same factors working in the opposite direction. 

 The volume of the blood remaining the same, certain 

 drugs are capable of causing vascular contraction, as 

 epinephrine, which, uninterfered with, would cause an 

 enormous rise in pressure. Injecting this drug into the 

 circulation does cause a rise in the pressure, but when 

 this reaches a certain level, the heart is slowed, in or- 

 der to prevent an undue strain. On the other hand, 

 when the vessels are caused to dilate by nitrites, the 



