FLOW OF BLOOD THROUGH THE ARTERIES 1049 



of the column of blood between it and the heart. The following figures were 

 found by von Kries : 



Finger : Mm. H 2 O Distance of finger 



below head 



328 . . mm. 



329 . . 205 mm. 

 513 . . 490 mm. 

 738 . . 840 mm. 



Ear : 20 mm. Hg. 



Gums of Rabbit : 33 mm. Hg. 



Frog's Web (Roy) : 100-150 mm. H 2 O. 



Capillary venous pressure of brain (Hill) : 



(1) Animal in horizontal position : 10 mm. Hg. 



(2) feet-down position : zero or less. 



(3) During strychnine convulsions : 50 mm. Hg. 



Owing to the fact that a varying and unknown resistance that 

 of the arterioles lies between the capillaries and the arteries, the 

 pressure in the capillaries must stand in much closer relationship to 

 that in the veins than to that in the arteries. One cannot therefore 

 argue that a fall of 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 direc- 

 tions 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 deter- 

 mined 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. 



