268 HUMAN PHYSIOLOGY 



Like the pressure in the arteries it depends upon the two 

 factors 



1st. Force of inflow. 



2nd. Resistance to outflow. 



(a) Variations in the Force of Inflow. The capillary 

 pressure may undergo marked local changes through the 

 vaso-motor mechanism. Wherever the function of a part is 

 active, dilatation of the arterioles and an increased capillary 

 pressure exists, and, when the influence of vaso-dilator nerves 

 is withdrawn, the capillary pressure falls. 



But the capillary pressure may also be modified by 

 the heart's action, inasmuch as the arterial pressure, by 

 which blood is driven into the capillaries, depends upon 

 this. In cardiac inhibition not only is arterial pressure 

 lowered, but capillary pressure may also fall. In aug- 

 mented heart action both arterial and capillary pressure 

 are raised. 



(6) Variations in Resistance to Outflow. Normally the 

 flow from capillaries to veins is free and unobstructed ; but, 

 if the veins get blocked, or if the flow in them is retarded by 

 gravity, the capillaries get engorged with the blood which 

 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, causing 

 dropsy. 



It is, therefore, most important to distinguish between 

 high capillary pressure from dilated arterioles or an 

 active heart, and high pressure due to venous obstruc- 

 tion. 



A condition very similar to that described, but producing 

 a capillary pressure high relatively to the pressure in the 

 arteries though not absolutely high is seen in cases of 

 failure of the heart, 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 the capillary pressure becomes high in relationship 



