THE CIRCULATION 



269 



to the arterial pressure ; the blood is not forced through these 

 channels, and congestion of the capillaries and dropsy may 

 result. 



The influence of gravity plays a very important part on 

 the capillary pressure, since it has so marked an influence 

 on the flow of blood in 

 the veins. When, through 

 heart failure, the blood is 

 not properly sucked up 

 from the inferior extremi- 

 ties, this increased pressure 

 becomes very marked in- 

 deed. 



But the pressure in the 

 capillaries may also to a 

 certain extent be varied by 

 the withdrawal of water 



from the body, as in pur- Fia 127. The Changes in Blood Pressure 

 ,. -,. . in the Capillaries produced by increas- 



gation or diuresis, or by ing the ^ rterial ; ressure .5 , 



the addition of large quan- and by obstructing the venous flow 



tities of fluid to the blood. 7 f Arteries ; & Cnpii- 



.,,, . , lanes ; V. , Veins. 



I he venous system is, how- 

 ever, so capacious that very great changes in the amount 

 of blood in the vessels may take place without materially 

 modifying the arterial or capillary pressure while affecting 

 temporarily the venous pressure. 



IV. VENOUS PEESSURE. 



In the veins the force of inflow is small; the resistance 

 to outflow is nil. Hence the pressure is small, and steadily 

 diminishes from the small veins to the large veins entering 

 the heart (Fig. 127). 



The venous pressure may be modified by variations in 

 these two factors. Constriction of the arterioles tends to 

 lower the venous pressure, dilatation to raise it. On the 

 other hand, increased heart's action, which so markedly 

 tends to raise arterial pressure, diminishes the pressure in 

 the larger veins, because the blood is thus more rapidly 

 driven from veins into arteries, and because the heart, which 



