The Semi-lunar valves.
Dateline: 05/11/97
In previous articles, we have discussed the atrio-ventricular valves that guard the junction between the atria and ventricles, directing blood flow in one direction inside the heart. The semilunar valves (or arterial valves) are the other set of valves inside the heart that guard the junction between the ventricles and the great arteries - the aorta and the pulmonary artery.
There are two semi-lunar valves. The aortic valve is located between the left ventricle and the aorta, and prevents blood that is pumped out of the left ventricle from leaking back into the chamber when the heart relaxes. The other is the pulmonary valve that is located between the right ventricle and the pulmonary artery, which functions to keep blood flowing in one direction only away from the right ventricle and into the lungs.
The semi-lunar valves, as the name implies, have cusps (or leaflets) that are half-moon shaped. Each valve has three such cusps that are attached to the wall of the great artery. A cusp is made up of thin fibrous tissue that is almost translucent. Each cusp has a margin that is attached to the wall of the aorta or pulmonary artery and another free margin. The cusps are positioned so that a portion of their free margin overlaps the adjacent cusp. This mechanism is of great importance in maintaining valve competence and preventing backwards leak of blood back into the ventricle. The point where two adjacent cusps meet at the wall of the great artery is called a commissure. Since there are three cusps, a semi-lunar valve normally has three commissures.
The aortic cusps are named according to the coronary artery which arises above it. There is a right coronary cusp (above which the right coronary artery arises), a left coronary cusp (with the left artery above it), and a non-coronary cusp which is unrelated to a coronary artery. The pulmonary valve also has three cusps, named according to their position - anterior, left and right posterior cusps.
In organization, the semi-lunar valves are simpler than the atrio-ventricular valves. Whereas the latter have papillary muscles and chordae tendineae, and require integrated functioning of the different components, the semi-lunar valves are elegant in their simplicity. Considering the enormous strain to which they are subject (the aortic valve supports a pressure of around 80 mm.of Hg), it is amazing how such a uncomplicated structure can work as well as it does. The structure of the semi-lunar valves has been likened to that of a suspension bridge, where the stress is borne by the struts of fibrous tissue at the commissures of the valve.
While normally each valve has three cusps, in some birth defects the number of cusps can be abnormal. When only two cusps are present, the valve is called a bicuspid valve. The birth defect called bicuspid aortic valve is one of the commonest cardiac anomalies. Early in life, this does not affect the heart's function because there is almost normal blood flow. However, the increased turbulence produced takes it's toll over time, and calcium is deposited on these valves producing narrowing of the orifice after around 20 years. This results in a disease called aortic stenosis.
Various diseases can also affect the semi-lunar valves making them narrow or leaky (regurgitant). In future articles, we will discuss in greater detail these diseases of the semi-lunar valves.
