354 THE MECHANICS OF THE CIRCULATION, HEMODYNAMICS 



tubing is under the greatest possible elastic tension and subjects the fluid within 

 to the steady pressure of its recoiling walls. Each compression of the syringe 

 increases the pressure slightly, while during the interims it is somewhat decreased 

 owing to the continuous escape of fluid. It is to be emphasized, however, that this 

 system must always be kept in a hyperfilled condition, otherwise the flow cannot 

 remain constant. 



Fig. 185. — Simple Schema to Illustrate the Factors Producing a Constant Head of 

 Pressure in the Arterial System. 

 o, A syringe bulb with valves, representing the heart; 6, glass tube with fine point 

 representing a path with resistance alone, but no extensibihty (the outflow is in spurts 

 synchronous with the strokes of the pump) ; c, outflow with resistance and also extensible 

 and elastic walls represented by the large rubber bag, e; the outflow is a steady stream due 

 to the elastic recoil of the distended bag, e. (Howell.) 



CHAPTER XXXI 

 BLOOD PRESSURE 



The Factors Responsible for Blood pressure. — In order to prove 

 that the blood flows from the arteries into the veins and thus completes 

 the circuit of the body, Harvey placed loose ligatures upon an artery 

 and neighboring vein and raised them gently out of the wound until 

 their lumina became fully constricted. It was then found that the 

 central end of the artery and the distal end of the vein were highly 

 distended, while their other ends were collapsed. If the walls of the 

 distended segments were then pierced with the point of a needle, the 

 blood spurted out in fine jets, but with a much greater force from 

 the artery than from the vein. The same observation was made during 

 capillary bleeding, because the blood oozes from these opened blood- 

 vessels in small droplets which presently coalesce to form a sheet-like 

 covering over the injured area. These and other observations read- 

 ily prove that the blood is held in the vascular system under a certain 

 pressure. 



The term blood pressure is often used to denote the general pres- 

 sure existing in the vascular system, while at other times it is intended 

 to indicate merely the pressure prevaihng in the arterial channels. 

 This ambiguity mav easily be avoided by making specific reference to 



