204 CIRCtTLATIOK 



Regulation of Blood Flow. 



Heart control. — The amount of blood which passes 

 through a given part of the circulatory s^'stcm at an}' one 

 time is, of course, regulated primarily by the heart beat. 

 This beat, which is produced by rhj^hmic contractions of 

 the heart muscles, is ordinarily very regular. Its rate, 

 however, is subject to certain external influences. For 

 example, a decrease in blood pressure in the arteries is 

 associated with an increase in heart rate and vice versa. 

 Muscular exercise also increases the heart rate. Even so 

 little an effort as a change in position is sufficient to change 

 the rate, that of a standing position being higher than 

 that of a sitting or reclining one. 



The purpose of these changes in rate is readily under- 

 stood when we consider that tissues which are acti\'e are 

 using up more food and creating more waste than still 

 tissues, and consequently require a greater flow to supply 

 this demand. 



Mental states and emotions, blood content, and tem- 

 perature, all affect the rate of heart beat. Some lower 

 this rate, others increase it. Some, like temperature, cause 

 an increase up to a -certain limit, and then above that 

 limit produce a decrease. Physicians take advantage of 

 these factors and introduce into the blood certain drugs 

 which may accelerate or lower the rate of beat, and thus 

 can control it to a certain extent in cases of disease. 



The manner in which all these special conditions are 

 able to affect the heart is through what ■\\e call nerve 

 action. The muscle walls of the heart are connected by 

 nerves to other parts of the body, and when one set of 

 nerves is stimvilated by any of the above means, the nerve- 



