The heart consists of four valves to control the flow of blood.  Each valve has the same basic structure although each one is unique down to fine detail.

  •        ATRIO-VENTRICULAR = Tricuspid and Mitral (Bicuspid)
  •        SEMI-LUNAR = Aortic and Pulmonary

The atrio-ventricular (AV) valves prevent the backflow of blood from the ventricles to the atria during systole (contraction). The valves are held in place by Chordae Tendinae, bands of fibrous tissue that attach to the cusps of each valve and to the papillary muscles located in the walls of the ventricles. However it is not the chordae tendinae and the papillary muscles that are responsible for the opening and closure of the valves, but the pressure gradient created across them.

During diastole (relaxation) of the ventricles, the AV valves are open allowing the ventricles to fill with blood from the atria. As the ventricles fill, the intra-ventricular pressure rises and as they enter systole, the AV valves are forced to close due to the pressure gradient. The closure of these valves creates the ‘lub’ sound or S1 phase of the heart sound. On the right side of the heart the AV valve is called the Tricuspid valve. On the left, it is called the Mitral or the Bicuspid valve.

  •    Mitral valve allows blood to flow from the left atrium to the left ventricle. It has two cusps (bicuspid).
  •   Tricuspid valve allows blood to flow from the right atrium to the right ventricle and has three cusps.

Structure of the Heart's Valves: Bicuspid and Tricuspid

The rising ventricular pressure forces the semi-lunar valves to open. These include the aortic valve and the pulmonary valve leading to the aorta and pulmonary trunk respectively. After ventricular systole and the ventricles relax again, the pressure drops rapidly and the semi-lunar valves close. Closure of these valves creates the second heart sound (S2) or ‘dub’ sound of a heartbeat.

  •  Aortic valve controls blood flow from the left ventricle to the Aorta.
  •  Pulmonary valve controls blood flow between the right ventricle and the pulmonary artery


image showing the structure of an open and a closed valveArrow depicting the direction of flow of blood

Images (own work)