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TEXT-BOOK OF PHYSIOLOGY 



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 converts 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 capillaries. This is accomplished in the follow- 

 ing manner: With each contraction 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 contraction, 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 contraction the elastic 

 walls recoil and propel the blood toward the 

 capillaries. The intermittent 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 

 characteristics the flow of a fluid through a 



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

 cular muscular fibers of the middle pump; that is, the intermittent movement im- 

 coat. d. The outer coat. (Lan- , j , , i i , i , i 



dois and Stirling ) parted, by tne neart is not so completely con- 



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 vessel 

 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 amount 

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

 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 in- 



