134 



HUMAN PHYSIOLOGY 



CHAP. 



^ 

 Aorta 



-1 y&tae 



flow is so great that the blood makes one circulation 

 from the heart to the tissues and back to the heart 



in about twenty-two 

 seconds. It takes 

 the blood longer to 

 go to the toes and 

 fingers than to parts 

 of the body nearer 



FIG. 72. Showing how the blood vessels com- the heart. The eil- 

 pare in area (Zuppke). , , , /- / / / 



tire quantity of blood 

 passes through the heart about three times per minute. 



100. What makes the Blood Move? There are a 

 number of different causes which together keep the 

 blood in constant motion. The most important cause 

 is the contraction of the muscles of the heart. When 

 the strong, thick muscles of the left ventricle contract, 

 the blood is pushed into the aorta with great force. 

 The continued pumping of the heart keeps forcing 

 more and more blood into the aorta and thus pushing 

 it along through all the blood vessels. The elastic 

 walls of the arteries aid the heart very much in forcing 

 the blood along. 



The heart works like an automatic pump. There 

 are two nerves going to the heart which control and 

 regulate its activity. One of these stimulates the 

 muscles of the heart and makes them work faster, as 

 when we are running hard ; the other checks the mus- 

 cular action and makes the heart beat slower. These 

 two nerves act like the governor of an engine. 

 They determine the rate of the heart beat, and so the 



