This 3D animation from The Human Atlas explains how the valves of the heart work.

The heart is a beating muscle that pumps blood throughout the body. Inside the heart, four valves open and close in precise sequence to keep the blood moving in the appropriate direction.

Two of the hearts valves control the flow of blood from the upper chambers of the heart, or atria, to the lower chambers, or ventricles. Two other valves control the flow of blood from the ventricles to the lungs and body. As the four valves open and close, they create two distinct "heart sounds" commonly referred to as the heartbeat.

During a normal heartbeat, oxygen-poor blood returning from the body fills the right atrium, which then contracts and pumps blood through the tricuspid valve and into the right ventricle. Next, the right ventricle contracts and pumps blood through the pulmonic valve and into the pulmonary artery. The pulmonary artery transports the blood to the lungs to receive oxygen.

At the same time, oxygen-rich blood returning from the lungs enters the left atrium, which pumps the blood through the mitral valve and into the left ventricle. As the left ventricle contracts, blood is pumped through the aortic valve and into the aorta, where it is distributed to the body.

Blood is routed through the heart by the valves. When the atria contract, the valves in these top chambers, the tricuspid valve and the mitral valve, open and allow blood to flow to the ventricles. When the ventricles contract, the tricuspid and mitral valve are forced shut while the ventricular outflow valves, the pulmonic and the aortic valves, are forced open by the ventricular pressure. Blood leaving the ventricles is prevented from flowing backwards by the tight seal formed by the cusps of the aortic and pulmonic valves.