144 OUR BODIES AND HOW WE LIVE 



stroke the ventricles pump about six ounces, or nearly fifty 

 teaspoonfuls, of blood. About eighteen pounds of blood are 

 moved every minute, or twelve tons every day. 



It is calculated that the total amount of daily work done by 

 the heart can be represented by what a man of average weight, 

 about one hundred and fifty pounds, would do in running up a 

 flight of forty steps forty times. While the cavities are filling 

 with blood, and its muscles are relaxed, the heart has a brief 

 rest ; otherwise, it could not keep up its patient and tireless 

 pumping of over four thousand tons of blood every year, from 

 birth to death. 1 



202. The Blood Vessels that enter and leave the Heart. 

 The aorta, the largest artery in the body, springs from the 

 left ventricle. It carries the bright, pure blood from the 

 heart to all parts of the body. 



Four pulmonary veins open into the left auricle. Two of 

 these veins come from the right lung and two from the 

 left lung. They bring back to the heart the blood which 

 has been combined with oxygen in the lungs. 



Two of the largest veins in the body, call-ed the superior 

 vena cava and the inferior vena cava, open into the right auricle. 

 These great veins pour into the right auricle the dark blood 

 which has been collected in various parts of the body by 

 the smaller veins (Fig. 95). 



The pulmonary artery springs from the right ventricle. 

 Soon after leaving the heart, it splits into two pipes ; one 

 goes to the right lung, the other to the left lung. This 

 artery carries from the heart to the lungs the dark, impure 

 blood which has been brought to it by the great veins. 

 Its entrance is guarded by the three semilunar valves. 



1 It should be remembered that the two auricles contract at the same time 

 and then the two ventricles. Then comes a pause, or state of rest, after 

 which the auricles and ventricles contract in the same order as before. 



