orce you to breath so that you can continue to live.William Harvey discovered that blood circulates throughout the body. He could not explain, however, the motion of the heart that makes circulation possible. As far as he was concerned, that could be comprehended only by God (3). However, more than three hundred years later we now have a vast knowledge of how the heart works. Each side of the heart performs a different pumping job. The right side takes blood from the body and pumps it to the lungs. The left side of the heart collects blood from the lungs and pumps it to the body. Blood goes through the right side of the heart, to the lungs, and then to the left side of the heart where it is once again pumped through the body (World Book H 139). Blood flows into the right atrium through two large veins, called the superior and the inferior vena cavas (139). The superior vena cava carries blood from the head and arms. The inferior vena cava carries blood from the trunk and legs. Blood from the body fills the right atrium. The right atrium then contracts, squeezing blood through the tricuspid valve into the ventricle. The tricuspid valve is made of three little triangular flaps of thin, strong fibrous tissue. The valve allows blood to flow into the ventricle but does not allow it to flow back out in the same direction. The ventricle is at first relaxed, but it contracts when it is filled with blood. The resulting pressure closes the tricuspid valve and opens the semilunar valve, which is between the ventricle and the pulmonary artery. Blood gushes through the semilunar valve and into the pulmonary artery, which leads to the lungs. The semilunar valve gets its name because its three flaps are in the shapes of a half moon (139). Blood flows from the lungs back into the left side of the heart through the four pulmonary veins and into the left atrium. The left atrium then, like its neighbor to the right, then contracts, squ...
