MONICA SCHROEDER / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY MONICA SCHROEDER / SCIENCE SOURCE / SCIENCE PHOTO LIBRARY
Diagram illustrating the paths of blood flow through the heart. Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium. The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium. Blood flows from the atria into the ventricles through the tricuspid valve and the mitral valve. When the ventricles are full, the valves shut. This prevents blood from flowing backward into the atria while the ventricles contract. When they do, blood leaves the heart through the pulmonic valve and the aortic valve.
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