166 APPLIED PHYSIOLOGY 



pressure tends to be maintained at a very constant 

 level ; but they are of sufficient importance to make it 

 necessary to exclude them when making blood-pressure 

 estimations in patients. 



Regulation of Blood-pressure. 



The constancy of the arterial blood-pressure is attained 

 by the control exercised over it by the general vaso- 

 motor centre, which exerts a continuous tonic influence 

 upon the muscular coat of the arterioles. By the agency 

 of this centre and its nervous connections the general 

 blood-pressure is kept fairly constant, whilst at the 

 same time the varying local needs of the organs or 

 tissues for an increased blood-supply are provided for, as 

 will be described more fully when we come to speak of 

 the distribution of blood in the body. 



When the pressure in the arteries tends to rise very 

 high, the heart is slowed through the influence of the 

 vagus, the reflex being probably started by the pressure 

 of the blood in the left ventricle ; and at the same time 

 its output is lessened, and thus the high pressure tends 

 to be counteracted. The diminution of output may in 

 these circumstances become so great that the ventricle 

 does not completely empty itself at each systole, with 

 the result that blood is dammed back into the lungs, 

 and thence into the veins, and all the symptoms of back 

 pressure result. Hence in conditions of high tension 

 one may get all the clinical signs of heart failure without 

 there being any defect of the mitral valve. 



If the rise of arterial pressure be very abrupt as, 



