520 THE OUTGO OF ENERGY 



to bursting. If more, the arteries would soon be 

 empty. As the quantity passing through the 

 capillaries and the arteries and veins in a unit of 

 time must thus be the same, it follows that where 

 the combined cross-section of the channel or 

 " bed " is small, the blood must flow faster than 

 where the cross-section is large. A river rushes 

 rapidly through a gorge, but moves sluggishly 

 where meadow-lands afford a wider channel. 

 Thus the blood flows with great velocity in the 

 great arteries, less rapidly in their branches, and 

 very slowly indeed in the capillaries, the com- 

 bined width of which is so great compared to 

 that of the arteries. And as the capillaries unite 

 into the smaller veins, and these into the larger 

 veins, the combined cross-section or bed becomes 

 ever smaller and the blood moves ever more 

 swiftly. Were the slow passage of the blood in 

 the capillaries due simply to friction, the blood 

 would move still more slowly in the veins be- 

 cause the retarding influence of the friction in 

 the veins would be added to that of the capillaries. 

 There is an inverse relation between the rate of 

 flow and the area of bed. 



