BLOOD VESSELS 



461 



needle connected with a reservoir of water and a manometer, 

 and estimating the capillary pressure by the pressure 

 required to drive the water into the subcutaneous tissue. 

 The assumption is made that the pressure in the tissues is 

 about equal to that in the capillaries. The movement of 

 the water may be determined by watching the movement of 

 a bubble of air in the tube (fig. 193). 



At the level of the heart the capillaries may be com- 

 pressed by a pressure of some 10 to 20 mm. Hg, but in the 

 leg about 90 mm. is required. 



It has already been shown that the pressure is less than 

 in the arteries and greater than in the veins. 



Fig. 193.— Method of Estimating Capillary Blood Pressure (see text). 



Like the pressure in the arteries, the pressure in the 

 capillaries depends upon the two factors — 



1st. Force of inflow. 



2nd. Resistance to outflow. 



It must be remembered that all the biood does not flow 

 through capillaries, but that in many situations arterioles 

 open directly into venules. Thus, in obstruction of the 

 capillaries, the blood may find its way through to the veins. 



1st. Variations in the Force of Inflow. — The capillary 

 pressure may undergo marked local cnanges through the 

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

 active, dilatation of the arterioles and an increased capillary 

 pressure exist, and, as has been already seen, the condition 

 of the capillaries plays a part. 



But the capillary pressure may also be modified by the 



