UJJ. 



S 



THE FLOW OF BLOOD THROUGH THE ARTERIES 975 



position of the part under investigation, *. e. with the hydrostatic pressure 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 Rabbits : 33 mm. Hg. 



Frog's Web (Roy) : 100-150 mm. H 2 0. 



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 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 



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

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

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

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



No part of the circulation however shows greater variations than the 

 capillary system. We must think of this as a vast irrigation system of 

 canals the greater part of which are closed under normal circumstances, 

 and open only when the chemical changes in the tissue require a large increase 

 in the supply of blood. In muscle the capacity of this irrigation system may 

 be increased 750 times during activity. A similar opening up of capillary 

 channels may be observed in the skin and connective tissues as a result of 

 irritation or injury. It seems probable that such changes will affect arterial 

 pressure by their influence on the total capacity of the vascular system (if of 

 wide enough occurrence) rather than by alterations thereby produced in 

 the peripheral resistance. 



