CHAPTER IV. 



THE BLOODVESSELS. 



Mechanics of the Circulation.* — The laws which regulate the 

 flow of fluids through elastic tubes apply with equal force 

 to the bloodvessels. 



An elastic tube converts an intermittent flow into a con- 

 stant one ; this it is enabled to do by the recoil of its coats 

 following on previous distension. The effect, however, of 

 the elastic reaction of the walls of the vessel is to diminish 

 the rate of flow, though at the same time it increases the 

 amount of fluid discharged. 



Applying these principles to the circulation, we can 

 readily see their important bearing in the body, and appre- 

 ciate the considerable economy effected in the work of the 

 heart through the blood being pumped into elastic rather 

 than rigid tubes. 



In a rigid tube as much fluid as is pumped in at one end 

 issues at the other, and the same will hold true for elastic 

 tubes unless they offer resistance to the flow. The effect of 

 peripheral resistance is to convert an intermittent into a 

 continuous flow. 



In the bloodvessels, the capillaries and smallest arteries. 

 from their small size and considerable area, offer great 

 resistance to the flow of blood through the arteries; this 

 causes the arteries to be distended, and the clastic recoil of 

 the stretched walls leads to the continuous flow through 

 them in spite of the intermittent action of the heart . 



The pressure exerted on the wall of a tube diminishes the 



* ' Physiological I'Iinmc-, by M'Gregor-Robertson, lias helped me 

 considerably with 1 1 1 * - mechanics of the circulation. 



