FLOW OF BLOOD THROUGH THE ARTERIES 931 



arterial pressure necessarily involves a fall of capillary pressure in all parts 

 of the body. We can only judge of changes in the capillary pressure by 

 taking simultaneously the pressures in both the afferent and efferent 

 vessels. If these both rise or fall together we may be certain that the 

 capillary pressure also rises or falls. Where the arterial and venous 

 pressures move in opposite directions it is difficult to say what alterations, 

 if any, will be produced in the capillary pressure. 



The resistance to the flow of blood through the capillaries is determined 

 by the internal friction, i.e. the viscosity of the blood ; this varies in 

 different animals between three and five times that of water. It has been 

 calculated that the fall of pressure undergone by the blood in passing 

 through any given capillary area is only about 20 to 60 mm. of blood, and 

 at the most is never more than 150 mm. blood, i.e. about 10 mm. Hg. This 

 bears out the conclusion to which we have already come, viz. that the 

 chief seat of the resistance in the vascular system is the arterioles, and it 

 is in this region that the chief fall of pressure occurs. 



