THE CIRCULATION 163 



of fluid is quickly drained off into the lymph spaces 

 or accommodated in the large veins of the abdomen. 

 Transfusion, therefore, may raise a depressed blood- 

 pressure to the normal level, but it cannot do much 

 more than this, and, like venesection, its effect tends to 

 be very transient. 



There is but very little difference between the pressure 

 in the large arteries near the heart and that in the 

 smaller peripheral arteries. It is only when the 

 arterioles themselves are reached that a rapid and 

 notable fall of pressure occurs. Hence it is immaterial 

 which artery is selected for the clinical estimation of the 

 blood-pressure, and the brachial, which is the artery 

 usually chosen, will afford quite a trustworthy indication 

 of the aortic pressure. 



Variations in Blood-pressure. 



The general arterial blood-pressure tends, like the 

 body temperature, to be maintained at an extraordinarily 

 constant level, and such variations as do occur are more 

 in an upward direction than a downward i.e., abnor- 

 mally high pressures are much commoner than abnor- 

 mally low ones. It would appear, indeed, that the 

 normal degree of pressure is pretty near the minimum 

 at which life can be constantly sustained, and every 

 effort is made to uphold it. 



The normal systolic pressure in a healthy young adult 

 male is about 120 millimetres Hg, and the diastolic 

 pressure about 100. In women they are about 10 per 

 cent, less, and in children the systolic pressure may be 



112 



