There are four valves within the heart that regulate the flow of blood between the heart’s four chambers. These valves are made up of either two or three flaps (cusps) that swing open to allow blood to flow through and close tightly to prevent blood backflow or leakage. The valves work similarly to a gate that opens and closes for the passage of blood.

The valves are synchronized to open and shut in the same rhythm as the systolic (contraction) and diastolic (relaxation) phases of the cardiac cycle. It is this opening and shutting of the valves that make the characteristic “lub-dub” sound of the heartbeat.

There are two valves on each side of the heart. The valves on the right side of the heart direct the flow of oxygen-poor blood as follows:

• Tricuspid valve. This three-cusp valve is located between the upper-right chamber (right atrium) and the lower-right chamber (right ventricle) of the heart. It is attached to papillary muscles on the ventricular wall by string-like chords called chordae tendineae. During the diastolic (resting) phase, the tricuspid valve opens, and oxygen-poor blood empties from the right atrium into the right ventricle. The valve then closes during the systolic phase to prevent blood from leaking backward into the right atrium.
  
  
• Pulmonic valve. The pulmonic valve, which also has three leaflets, is located between the right ventricle and the pulmonary artery, which carries oxygen-poor to the lungs for fresh oxygen. After the right ventricle contracts (the systolic phase), blood is pushed through the pulmonic valve and into the pulmonary artery. The valve then closes during the diastolic phase to prevent blood from leaking backward into the right ventricle. The pulmonary artery carries the oxygen-poor blood to the lungs.

The two corresponding valves on the left side of the heart direct the flow of oxygen-rich blood as follows:

• Mitral valve. This valve is located between the upper-left chamber (left atrium) and the lower-left chamber (left ventricle) of the heart. It is made up of two cusps that are attached to the left ventricle near the left atrium. When the left atrium is filled with oxygen-rich blood from the lungs, the cusps swing open to allow blood to flow from the left atrium to the left ventricle. This occurs during the diastolic phase. Afterward, the valve closes, billowing only slightly back into the left atrium at the beginning of the systolic phase.
  
  
• Aortic valve. This valve is located between the left ventricle and the aorta, which is the main artery that carries oxygen-rich blood to the rest of the body. During the systolic phase, the left ventricle pumps the oxygen-rich blood through the three-leaflet aortic valve and out through the aorta. Afterward, the valve closes, billowing only slightly back into the left ventricle at the beginning of the diastolic phase.

The pulmonic and aortic valves open simultaneously during the systolic (muscle contraction phase), and the tricuspid and mitral valves open simultaneously during the diastolic (muscle relaxation phase). In a healthy heart, the valve leaflets are wide enough to permit the free flow of blood and can close tightly enough to prevent leakage and/or backflow after each batch of blood has been pumped through.

Narrowed valves are referred to as valvular stenosis, and improperly closing valves that allow blood to leak back in the wrong direction are referred to as valvular regurgitation. Valves that are permanently closed since birth are referred to as valvular atresia.