Blood flowing from any part of the body, including the left hand, enters the right side of the heart through either the superior vena cava or the inferior vena cava. Since blood from the left hand would be coming to the heart from the upper part of the body, it would enter via the superior vena cava. From the superior vena cava, the blood would be passed onto the right atrium. When the right atrium contracts, blood is pushed through the tricuspid valve and into the right ventricle. From the right ventricle, blood is pumped through the pulmonary valve and into the pulmonary artery. The blood passes by the lungs where it picks up oxygen, which is needed so the oxygen can be delivered to cells. Cells need oxygen so they can undergo cellular respiration and produce an energy source called ATP (adenosine triphosphate). The ATP helps the cells and organism in which the cells are located perform daily functions needed for survival.
The oxygenated blood then returns to the left side of the heart via the pulmonary veins. From the pulmonary veins, the blood travels to the left atrium. As the left atrium contracts, blood is pushed through mitral valve and through the left ventricle. The left ventricle pushes blood through the aortic valve and into the aorta. This is a key step in blood circulation. The aorta is considered to be the artery that distributes blood to all other parts of the body.
Once the blood is distributed to where it is needed and its oxygen is depleted, then the cycle starts over.
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