THE CIRCULATION OF THE BLOOD 159 



it must be remembered that these are usually people who 

 fail to take proper exercise or sufficient sleep and rest; the 

 indigestion from which they too frequently suffer is more 

 often attributable to these causes than to the fact that the 

 digestive organs are deprived of their proper blood supply. 



21. The mechanism of the regulation of the flow of blood. 

 Having thus considered exactly what takes place in the circu- 

 lation during some of the more important events of daily life, 

 we may next inquire briefly into the physiological mechanism 

 by which these adjustments are secured. Its most important 

 features are the regulation of the inflow from the heart into 

 the arterial reservoir and the regulation of the outflow 

 through the arterioles and capillaries of the organs. These 

 two must be adjusted to each other in order that the reser- 

 voir may remain full and thus the driving force for the flow 

 through the organs be maintained. We shall go into the de- 

 tails of this very beautiful but complicated mechanism only 

 far enough to enable the student to appreciate certain prin- 

 ciples of fundamental importance in the practical conduct 

 of life. 



22. The regulation of the pumping action of the heart. The 

 amount of blood which the heart pumps varies considerably 

 from time to time. At times it may be as low as three 

 quarts a minute and at other times as high as twelve quarts, 

 the quantity being largely determined by the drain made at 

 the time upon the arterial reservoir. It will be seen at once 

 that this involves a wide range of adjustment. 



The beat of the heart is primarily due to events which 

 take place within the heart itself. We have seen that this 

 beat is a muscular contraction. But the cardiac muscle dif- 

 fers from the skeletal muscle in that it does not require an 

 impulse from the central nervous system to throw it into 

 activity. When the heart is cut off from connection with 

 the rest of the body, it continues to beat for a time and, if 

 supplied with warm blood, it may be kept beating for hours. 



