How the heart works
The heart is a hollow muscular pump that takes the blood it receives and pumps it out. The heart pumps a little over one gallon of blood each minute, usually beating 50 to 80 times per minute.
The blood, depleted of oxygen, flows back to the heart through two main veins. The venous blood from the head and arms flows in the superior vena cava, and the venous blood from the legs and the trunk flows in the inferior vena cava. These large vena caval veins enter the top right chamber of the heart, the right atrium. The right atrium fills and then pumps the blood through the tricuspid valve into the bottom right chamber, the right ventricle. The right ventricle then pumps the blood through the pulmonic valve into the pulmonary artery that takes the venous blood to the lungs where oxygen is added. The oxygenated blood returns via the four pulmonary veins to the top left chamber, the left atrium. The left atrium then pumps it through the mitral valve to the lower left chamber, the left ventricle. This most important chamber, the left ventricle then pumps the blood through the aortic valve into the aorta, the main artery that divides into smaller arteries going to every part of the body. The blood eventually goes into the smallest vessels, the capillaries where the tissues of the body absorb the oxygen they need. The blood, again depleted of oxygen, finds its way to the veins completing the circuit.