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When you listen to your heartbeat through a stethoscope ("lubb-dubb lubb-dubb"), you hear the sound of your heart valves closing. Although your heart has four valves, the valves open and close two at a time. That's why you hear only two thumps (one "lubb-dubb") per heartbeat, rather than four. Your heart valves keep blood flowing in one direction through your heart, just like the one-way valves in your home's plumbing. They open to let blood flow through, and then close to prevent blood from flowing back the way it came. When a valve closes, flaps of tissue on the valve close tightly together to create a seal. These flaps of tissue are called leaflets. Your heart has four valves. Blood flows through each valve one time on its way through your heart. The four valves can be grouped by their job: Atrioventricular — Atrioventricular valves control blood flow between your heart's upper and lower chambers .The valve between the right atrium and the right ventricle is called the tricuspid valve. The valve between the left atrium and the left ventricle is called the mitral valve. Semilunar — Semilunar valves control blood flow out of your heart. Blood flows out of the right ventricle to the lungs through the pulmonary valve. Blood flows out of the left ventricle to your body through the aortic valve.