8.5. Valves of the Heart

Four Main Valves

1. Bicuspid (also called mitral) valve
Between the left atrium and left ventricle. It has two cusps (note: it is not shown in the above diagram).

2. Tricuspid valve
Between the right atrium and right ventricle. It has three cusps (note: it is not shown in the above diagram).

3. Aortic valve
A type of semilunar valve with three cusps.

4. Pulmonary valve
A type of semilunar valve with three cusps.

The bicuspid and tricuspid valves have already been discussed, so no more will be said about them here. The aortic valve lies at the junction of the left ventricle/ascending aorta and it has three cusps: one posterior cusp and two anterior cusps. The two anterior cusps are of particular importance because just above each one lies an aortic sinus (one left and one right). These right and left aortic sinuses open into the right and left coronary arteries. After the left ventricle contracts, the sinuses and coronary arteries fill with blood to supply the heart muscle (myocardium). The posterior cusp does not give rise to any coronary vessels. Tips and tricks to help remember these cusps can be found at the end of Section 8. The pulmonary valve is less remarkable. The cusps of both the aortic valve and the pulmonary valve can be easily labelled. Remember that they basically appear as triangles, with one side fixed to the aortic wall. The two free borders of each cusp are called ‘lunules’ and the tip/apex of the valves can be called ‘nodules’. As you might expect, the valve is fully closed when both the lunules and nodules of each cusp are in contact with those of the other cusps. The nodules often have additional cartilage to strengthen them.

Function of the Valves

The aortic and pulmonary valves exist to prevent the backflow of blood into the heart after ventricular contraction (also called ventricular systole). During relaxation (also called ventricular diastole), blood is bounding through the aorta (and pulmonary trunk), but the elasticity of the aorta results in some blood flowing backwards. However, the cusps fill with this blood are are forced shut to prevent backflow into the left ventricle. The huge increase in aortic pressure also helps to close the valve. So, in summary, the bi- and tricuspid valves prevent backflow from the ventricles into the atria.