THE CIRCULATION OF THE BLOOD 



329 



adapt themselves to the variations in the volume of blood discharged from 

 the ventricle at a single beat or in a unit of time. The elasticity also con- 

 verts the intermittent movement of the blood imparted to it by the heart 

 as it is ejected from the ventricle, into a remittent movement in the arteries 

 and finally into the continuous and equable movement observed in the capil- 

 laries. This is accomplished in the following manner: With each contrac- 

 tion of the left ventricle more blood is ejected into the aorta than the arteries 

 can discharge into the capillaries and veins during the time of the contrac- 

 tion, owing to the small caliber and friction of the arterioles. The portion 

 not so discharged exerts a lateral pressure against the walls of the arteries 

 which at once dilate until a condition of equilibrium is established between 

 the pressure from within and the elastic reaction of the arterial walls from 

 without. With the cessation of the contrac- 

 tion the elastic walls recoil and propel the 

 blood toward the capillaries. The intermit- 

 tent action of the heart is thus succeeded by 

 the continuous reaction of the arterial wall. 



As the blood advances toward the periph- 

 ery of the arterial system and larger amounts 

 pass into the capillaries, both the distention 

 and the elastic recoil diminish, and by the time 

 the blood reaches the capillaries its intermit- 

 tency of movement has been so far obliterated 

 by the elastic recoil that as it enters the capil- 

 laries the movement becomes equable and con- 

 tinuous. In this manner the arteries modify 

 and change the character of the blood flow 

 and in part adapt it for the conditions of the 

 blood flow in the capillary vessels. 



In youth the arterial walls are highly dis- 

 tensible and elastic; in advanced years they 

 are frequently relatively rigid and inelastic, 

 and in consequence the flow of blood toward 



and into the capillaries approximates in its I49 ._ CoATS OF A SMALL 



characteristics the flow of a fluid through a j^wx. a. En dot h'e Hum. b. 

 rigid tube under the intermittent action of a internal elastic lamina, c. Cir- 

 pump; that is, the intermittent movement im- 

 parted by the heart is not so completely con- dois and Stirling.) 

 verted into a continuous movement, and hence 

 the blood flows through the capillaries during the systole with greater velocity, 

 and during the diastole with less velocity, than is the case when the vesse 

 is normally elastic. For these and other reasons the tissues are not so well 

 nourished and hence their nutrition and functional activities decline. 



Contractility. The contractility permits of a variation in the amoi 

 of blood passing into a given capillary area in a unit of time. Normally ead 

 artery has a certain average caliber due to a given contraction of the muscle 

 coat Beyond this average condition the artery can pass in one direction or 

 the other by either a relaxation or increased contraction of the muscle coat. 

 During the functional activity of any organ or tissue there is need for an 

 crease in the amount of blood beyond that supplied during functional mac- 



