HUMAN STRUCTURE AND LIFE-ACTIVITIES 491 



of rubber tubing.) When the pump is worked as before, jets of 

 water will issue from the end of the glass tube, no matter how large 

 or small an opening is present. The same thing happens in an ordi- 

 nary force-pump, and in order to get a steady flow of water in our 

 city water systems an air-chamber near the pump is used. (Explain 

 action of air-chamber.) Obviously the elastic arteries afford a simple 

 way to get the effect of an air-chamber on a force-pump ; namely, 

 a steady flow of fluid through the small branches (capillaries) of the 

 system of tubes. 



In 414 it is shown that another advantage of elastic 

 arteries is in regulating the flow of blood to the organs. 



414. Regulation of Blood Flow. It often happens that 

 an increased activity of the organs demands a greater move- 

 ment of blood for carrying more food, oxygen, or excretions ; 

 or a decreased activity demands a lessened circulation. In 

 short, there is need of a regulated blood flow from the heart 

 through the organs. This is brought about in two ways; 

 (1) by control of the heart, and (2) control of the arteries. 



Control of the heart is through the nervous system, certain 

 nerves carrying to the heart impulses or " messages " which 

 increase the rate of its beat, while other nerves inhibit or 

 diminish the beat. 



Control of the arteries is likewise the function of certain 

 nerves. The elastic small arteries in all living organs are 

 usually somewhat constricted, thus offering some resistance 

 to arterial flow, and keeping up a continuous stream through 

 the capillaries and veins back toward the heart. This con- 

 striction or reduced caliber is produced by a contraction of the 

 muscle-fibers which are in the walls of arteries, and this con- 

 traction is under control of nerves. 



When an organ needs an increased blood-supply (e.g., 

 the stomach in digestion), it may be obtained by a nervous 

 reaction leading to a relaxation of the muscles of smaller 

 gastric arteries and an increased diameter. Similarly, the 

 application of hot water or alcohol to the skin leads reflexly 

 to an increased blood flow; but cold water produces the 



