CIKCULATION 



285 



cannot escape from them. This increased pressure in the 

 capillaries is very different from that caused by increased 

 inflow. The flow through the vessels is slowed or may be 

 stopped instead of being accelerated, and the blood gets 

 deprived of its nourishing constituents, loaded with waste 

 products, and tends to exude 

 into the lymph spaces, caus- 

 ing dropsy. 



It is, therefore, most im- 

 portant to distinguish be- 

 tween high capillary pressure 

 from dilated arterioles or an 

 active heart, and high pres- 

 sure due to venous obstruc- 

 tion. 



A condition very similar 

 to that described, but pro- 

 ducing a capillary pressure 

 high relatively to the pressure 

 in the arteries though not 

 absolutely high is seen in 

 cases of failure of the heart, 



FIG. 131. The Changes in Blood Pres- 

 sure in the Capillaries produced by in- 

 creasing the arterial pressure ------ , 



and by obstructing the venous flow 

 . A, arteries ; C, capil- 

 laries ; V, veins. 



when that organ is not acting sufficiently strongly to pass the 

 blood on from the venous into the arterial system. Here the 

 arterial pressure becomes lower and lower, the venous pressure 

 higher and higher, and along with this trie capillary pressure 

 becomes high in relationship 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 

 extremities, this increased pressure becomes very marked 

 indeed. 



3rd. But the pressure in the capillaries may also to a certain 

 extent be varied by the withdrawal of luater from the body, 

 as in purgation or diuresis, or by the addition of large quan- 

 tities of fluid to the blood. The venous system is, however, 

 so capacious that very great changes in the amount of blood 



