162 APPLIED PHYSIOLOGY 



the peripheral resistance are the most potent cause of 

 individual and pathological variations in blood-pressure, 

 especially of diastolic pressure. 



3. The Elasticity of the Arterial Wall. Without 

 this there would be no diastolic pressure at all, and the 

 flow would correspond to that through rigid tubes 

 i.e., the pressure would be alternately very high and 

 reduced to nil. In the rigid arteries of advanced 

 atheroma an approximation to this state of things 

 actually obtains. 



4. The Volume of Blood in Circulation. This 

 factor is not of so much importance as might be ex- 

 pected, owing to the action of compensating mechanisms. 

 If the total volume of the blood is lessened e.g., by 

 haemorrhage blood -pressure is temporarily lowered; 

 but it is quickly restored again by the power of the 

 bloodvessels to contract down and accommodate them- 

 selves to the amount of blood they contain, whilst at the 

 same time fluid tends to be withdrawn from the tissues. 

 It follows from this that venesection is but of temporary 

 use as a means of lowering tension. Conversely, if the 

 amount of the circulating blood be increased e.g., by 

 transfusion the blood-pressure is only raised for a short 

 time, and to a comparatively slight degree ; for the excess 



example, has been shown by Crile, who found that, by enclosing a 

 patient in a pneumatic rubber suit into which air can be pumped, 

 the blood-pressure could be controlled to the extent of 25 to 60 milli- 

 metres of Hg. He makes use of the pneumatic suit in order to 

 produce an artificial peripheral resistance in cases of shock, in 

 which the general vasomotor centre is completely paralyzed (see 

 1 Blood-pressure in Surgery '). 



