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 auricle when it is forced out of the ventricle. 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 contraction which forces the blood out into the arteries. Blood is kept on its course 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 contraction of the ventricle walls. [Illustration: The heart is a force pump; prove it from these diagrams.] [Illustration: I. Circulation in a fish. _G_, gills; _C_, capillaries of the body. Notice the two-chambered heart. II. The circulation in a frog. _L_, the lungs; _C_, the capillaries. Notice the heart has three chambers. What is the condition of blood leaving the ventricle to go to the cells of the body? III. The circulation in man. _H_, head; _A_, arms; _L_, lungs; _S_, stomach; _Li_, liver; _K_, kidney; _S.I._, small intestine; _L.I._, large intestine; _Le_, legs; _1_, right auricle; _2_, right ventricle; _3_, left ventricle; _4_, left auricle; _5_, dorsal aorta; _6_, vein to lungs. ] 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 likewise passes later through the left heart. There are two distinct systems of circulation in the body. The _pulmonary 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 circulation, the blood receiving in the lungs its supply of oxygen, and there giving up some of its carbon dioxide. The greater circulation is known as the _systemic circulation_; in this system, the blood leaves the left ventricle through the great dorsal _aorta_. A large part of the blood passes directly to the muscles; some of it goes to the nervous system, kidneys, skin, and other organs of the body. It gives up its supply of food and oxygen in these tissues, receives the waste products of oxidation while passing through the capillaries, and returns to the right auricle through two large vessels known as the _venae cavae_. It requires only from twenty to thirty seconds for the blood to make the complete circulation from the ventricle back again to the starting point. This means that the entire volume of blood in the human body passes three or four thousand times a day through the various organs of the body.[46]