CIRCULATION 33 



heart, beats more rapidly in the attempt to raise the 

 pressure back to the normal level. 



Occasionally, this compensatory action fails. Such is 

 the case in fainting or syncope. Here, the vessels, es- 

 pecially in the abdominal region, dilate, and insufficient 

 pressure is maintained to supply the brain properly with 

 blood, so unconsciousness occurs. In moderate hemor- 

 rhage, the loss of blood is compensated for in the man- 

 ner described, but when the loss has assumed very se- 

 rious proportions, the pressure falls and unconsciousness 

 results. It is obvious that this fall of pressure is con- 

 servative ; if it did not take place, great difficulty would 

 occur in regard to clotting. Therefore, it may be un- 

 wise to resort to measures aimed to raise the blood pres- 

 sure before the bleeding point has been located and the 

 hemorrhage checked. 



Diseases which harden the walls of small vessels, con- 

 verting them into inelastic tubes, have the effect of con- 

 stricting vessels, increasing resistance and raising blood 

 pressure. Many drugs affect blood pressure either, like 

 digitalis, increasing the power of the heart, or, like the 

 nitrites (amyl, sodium, etc.) by dilating peripheral 

 vessels and causing a fall of blood pressure. Others act 

 upon the nervous mechanism of the heart, which, 

 roughly, consists of a set of sympathetic fibers which 

 accelerate or augment the action of the heart and a set 

 of vagus or pneumo gastric fibers which slow or inhibit 

 its action. This subject will be discussed under the 

 nervous system. 



