170 



THE BLOOD AND ITS CIRCULATION 



The Heart in Action. — The heart is constructed on the same 

 plan as a force pump, the valves preventing the reflux of blood 

 into the auricles when it is forced out of the ventricles. Blood 

 enters the auricles from the veins because the muscles of that part 

 of the heart relax; this allows the space within the auricles to 

 fill. Almost immediately the muscles of the ventricles relax, thus 



allowing blood to pass into 

 the chambers within the 

 ventricles. Then, after a 

 short pause, during which 

 time the muscles of the 

 heart are resting, a wave 

 of muscular contraction 

 begins in the auricles and 

 ends in the ventricles, with 

 a sudden strong contrac- 

 tion which forces the blood 

 out into the arteries. 

 Blood is kept from flowing 

 backward by the valves, 

 which act in the same 

 manner as do the valves 

 in a pump. The blood is 

 thus made to pass into 

 the arteries upon the con- 

 traction of the ventricle 

 walls. 

 The Course of the Blood in the Body. — Although the two sides 

 of the heart are separate and distinct from each other, yet every 

 drop of blood that passes through the right heart also passes 

 later through the left heart. There are two distinct systems of 

 circulation in the body. The puVmonary circulation takes the 

 blood through the right auricle and ventricle, to the lungs, and 

 passes it back to the left auricle. This is a relatively short cir- 

 culation, in which the blood receives oxygen in the lungs and gives 

 up carbon dioxide. The greater circulation is known as the sys- 

 temic circulation; in this system, the blood leaves the left ven- 

 tricle through the great dorsal artery called the aor'ta. Through 



The heart compared with a force pump. 



