The human heart is a hollow, pear-shaped organ about the size of a fist. The heart is made of muscle that rhythmically contracts, or beats, pumping blood throughout the body. Oxygen-poor blood from the body enters the heart from two large blood vessels, the inferior vena cava and the superior vena cava, and collects in the right atrium. When the atrium fills, it contracts, and blood passes through the tricuspid valve into the right ventricle. When the ventricle becomes full, it starts to contract, and the tricuspid valve closes to prevent blood from moving back into the atrium. As the right ventricle contracts, it forces blood into the pulmonary artery, which carries blood to the lungs to pick up fresh oxygen. When blood exits the right ventricle, the ventricle relaxes and the pulmonary valve shuts, preventing blood from passing back into the ventricle. Blood returning from the lungs to the heart collects in the left atrium. When this chamber contracts, blood flows through the mitral valve into the left ventricle. The left ventricle fills and begins to contract, and the mitral valve between the two chambers closes. In the final phase of blood flow through the heart, the left ventricle contracts and forces blood into the aorta. After the blood in the left ventricle has been forced out, the ventricle begins to relax, and the aortic valve at the opening of the aorta closes.

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