PART III 



THE CIRCULATION OF THE BLOOD 



CHAPTER XV 



BLOOD PRESSURE 



The object of the circulation is to maintain through the tissues a sup- 

 ply of blood that is adequate to meet their demands for nutriment and 

 oxygen and to remove the effete products of their metabolism. The de- 

 mands vary according to the activities of the tissue, being particularly 

 variable in the case of such tissues as the muscular and the glandular. 

 In studying the physiology of the circulation we have therefore to bear 

 in mind two aspects of the problem: (1) the cause for the continuous 

 bloodflow, and (2) the mechanism by which alterations in this bloodflow 

 are brought about. 



If we open an artery we shall find that the blood escapes from it 

 under such a pressure that it is thrown to a height of about six feet, 

 that its outflow is proportional to the size of the artery, and that it pul- 

 sates. If, on the other hand, we open a vein, we shall find that the 

 blood wells out without any very evident pressure, and that it flows 

 in a continuous stream, its outflow being the same in a unit of time as 

 that of the artery, provided the two vessels are the only ones supplying 

 the particular area. The general conditions governing the bloodflow 

 are the same as those governing the flow of fluid through any system of 

 tubes. For example, in the city water mains it is known to every one 

 that the rate of outflow from any part of the system depends finally on 

 two factors: (1) the difference in pressure at the beginning and end of 

 the system, and (2) the caliber of the tube at the outlet. We may in- 

 crease the outflow either by raising the pressure at the beginning of the sys- 

 tem, the caliber of the outlet meanwhile remaining constant, or by main- 

 taining the pressure constant but increasing the caliber of the outlet. 



In the circulation of the blood, the difference in pressure at the be- 

 ginning and end of the circulation is furnished by the pumping action 

 of the heart, and the alteration of the caliber of the outlet is provided 

 for by the constriction or dilatation of the blood vessels. These simple 

 physical principles indicate the direction which a study of the circulation 



124 



