TRANSPORT OF MATERIALS 



145 



water, whereas that of the whole blood may be as much as four 

 or more times that of water. 



A high pressure in the arteries could only be maintained by the 

 heart if there were resistance to the outflow through the small 

 branches. This resistance is not due to friction between the blood 

 and the walls of the vessels, because the layer in contact with the 

 wall is stationary, but to that between successive layers of the 

 blood itself, extending to some distance from the wall. In a small 

 tube this distance is great enough to reach to the middle of the 

 lumen ; in a large one the greater part of the current may be all 



FlG. 9. To illustrate the relative magnitude of the region where internal 

 friction of the blood takes place in large and small arteries of equal total 

 sectional area. 



moving at the same rate, and experience no internal friction (Fig. 9). 

 Hence the chief situation of the resistance is to be found in the fine 

 branches of the arteries, the arterioles. A volume of blood flowing 

 through a large tube experiences friction only in a small part near 

 the walls. The same volume flowing through a number of smaller 

 tubes, of the same total sectional area, experiences friction through- 

 out its mass. We see how the peripheral resistance, and with it 

 the arterial pressure, may be increased by constriction of the 

 arterioles or decreased by their dilatation. 



The Regulation of Blood Supply. If the diagram of Fig. 7 

 be referred to, it will be realised that if all but one of the parallel 

 channels be made narrower, this one will receive more supply ; 

 because if the heart continues to beat with the same strength, the 



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