THE CIRCULATION 155 



The pressure of the blood in the capillaries is only 

 second in importance to the rate of flow, and upon it in 

 large measure the amount of lymph transudation de- 

 pends. The pressure in the capillaries is surprisingly 

 high, being often, according to Hill, above 100 millimetres 

 of Hg. The capillary wall is so slender that it would be 

 unable to withstand such a pressure were it not sup- 

 ported from without by the ' tension ' of the tissues. 

 The degree of this tension varies greatly in different 

 localities, being greatest in the lower limbs, and where 

 it is low increased transudation from the capillaries, or 

 even rupture of them from the strain of internal pressure, 

 is apt to occur. This is the reason for subconjunctival 

 dropsy and occasional ecchymosis as the result of the 

 high tension of chronic renal disease, for the 'tension' of 

 the subconjunctival tissue is very low. 



The pressure in the capillaries depends more upon 

 the venous than upon the general arterial pressure. 

 When the small arterioles are contracted the capillaries 

 are thereby shielded to some extent from the high 

 arterial pressure, and if the contraction be extreme the 

 capillary pressure may fall to zero. On the other hand, 

 if the pressure in the veins be raised it tells directly back 

 upon the capillaries, and the transudation in them is 

 increased. This is often a factor in the production of 

 dropsy, and it is taken advantage of to increase local 

 transudation in Bier's method of treatment. Dilatation 

 of the arterioles raises the capillary pressure, and if the 

 dilatation be local and the general blood-pressure high, 

 the capillary blood-pressure in the area concerned may 

 rise very greatly. This is the reason why haemorrhage 



