158 



ADVANCED PHYSIOLOGY 



less prominent beat is supposed to be caused by the closure of 

 the semilunar valves of the heart. 



RATE OF BLOOD FLOW 



With such a powerful organ as the heart 

 driving the blood about the body, the blood 

 current is fairly swift. The blood flows rapidly 

 in the large arteries, but as they branch, the 

 total area 'of arteries becomes greater, so that 

 it flows rnore and more slowly; just as a river 

 flows swiftly through a narrow gorge, but 

 more slowly when it spreads out into a 

 broad stream. In the large arteries near 

 the heart blood flows at the rate of about 

 sixteen inches per second. Farther from 

 the heart the rate is about nine inches per 

 second; in the smaller arteries it is much 

 slower; in the capillaries it flows not more 

 than -g 1 ^ inch per second. After passing into 

 the veins the rate increases and as the veins 

 merge into trunks and finally reach the 

 heart it is about as rapid as in the arteries 

 that leave the heart. It is in the capillaries 

 that the blood exchanges new food materials 

 for worn-out matter, and oxygen for carbon 

 dioxid; and to allow this to take place to ad- 

 vantage a very slow flow is necessary. The 

 length of time required for the blood to make 

 a complete circuit of the body is calculated at 

 twenty-eight seconds, requiring thirty-two to 

 thirty-four heart beats. 



THE VASO-MOTOR SYSTEM 

 Regulation of the Size of the Blood Vessels. 

 By changing the rapidity of the heart beat, the whole body 

 may be made to receive more or less blood than usual; but 



PIG. 89. THE 

 LOWEB LEG 



FROM BE- 

 HIND 



Showing arter- 

 ies. Many of 

 the muscles 

 have been 

 removed . 

 (Modi fied 

 from Tiede- 

 mann) 



