THE CIRCULATION OF THE BLOOD 147 



muscles and glands, the seat of very active chemical changes, 

 require more blood than a tendon ; and a gland requires 

 more blood during the process of secretion than during rest. 

 How is the supply of blood to the organs regulated to meet 

 their varying needs ? In the first place, some organs are 

 more vascular than others; those requiring a larger supply 

 of blood receive a greater number of arteries from the arte- 

 rial reservoir and have a closer network of capillaries. But 

 in addition to this, these smaller arteries contain circular 

 muscle fibers whose contraction diminishes the bore of the 

 tube. When an organ needs more blood the muscle fibers of 

 its small arteries relax, thus permitting the arterial tubes to 

 widen or dilate just as when we want the water to flow 

 faster from a faucet, we widen the outlet from the pipe by 

 turning the spigot a little further. When less blood is needed 

 the small arteries are caused to constrict, just as a spigot 

 may be partially turned off (see sects. 25-27). In this way 

 the flow of blood to any organ is regulated to meet the 

 varying needs of the organ in question. 1 



12. Secondary aids to the circulation. In the preceding 

 discussion we have seen that the cause of the flow of blood 

 through the organs is the difference of pressure in the two 

 reservoirs. We have further seen that this difference of 

 pressure is maintained by the heart beat in pumping blood 

 from the venous into the arterial reservoir. A moment's 

 consideration will show that anything which hastens the flow 

 of blood from the veins into the heart and so lowers pres- 

 sure within the veins would similarly aid the circulation, 



1 In order that the student may become more familiar with these funda- 

 mental hydraulic principles of the circulation, such questions as the follow- 

 ing should be answered : (1) What are the two principal factors whose 

 variations change the amount of arterial pressure ? Illustrate by an example 

 or model. (2) How would the dilation of all the arteries of the intestine 

 affect the general arterial pressure ? (3) What would be the effect upon the 

 amount of blood flowing through the skin under this condition ? (4) How 

 would dilation of the arteries of the skin affect the blood flow through 

 the brain? 



