The semilunar valves (SV) are two valve structures that sit between the right ventricle and the pulmonary artery and between the left ventricle and the aorta. The flaps of the valves open and close to allow the unidirectional flow of blood out of the heart, while preventing backflow of blood. The valve between the left ventricle and aorta is called the aortic valve and the valve between the pulmonary artery and right ventricle is called the pulmonary valve.
The heart has two types of valves: the atrioventricular (AV) and semilunar. The atrioventricular valves allow blood to pass from the atria to the ventricles, closing tight to block leakage of blood back into the atria. The atria are the top chambers of the heart that receive blood. The ventricles are the bottom two chambers that pump blood out of the heart, either toward the lungs to pick up oxygen, or out into the body. The semilunar valves open to let blood pass out of the ventricles. The right ventricle pumps into the pulmonary artery, which leads to the lungs, and the left ventricle empties into the aorta, a main artery that distributes oxygen rich blood to the rest of the body.
The semilunar valves are shaped like half moons and have three cusps, or flaps. Unlike the atrioventricular valves, they do not have tough cords of connective tissue called chordae tendineae to force the cusps open. Like valves in veins, both semilunar valves simply use pressure to determine when to open and close.
Blood circulates through the heart via differences in pressure between the atria and ventricles and due to contractions of the myocardium, or the muscular layer of the heart’s wall. As this latter process, called systole, happens in the ventricles, it causes the pressure in the ventricles to rise. When the pressure in the ventricle rises above the pressure within its corresponding artery, the semilunar valves open and release blood. Failure of the valve to open or close completely can cause complications that range from asymptomatic to debilitating.
Stenosis of the pulmonary or aortic valve occurs when the blood flow out of the heart is slowed by the valve’s failure to open completely. When the valves do not close fully, it causes pulmonary or aortic regurgitation, which allows blood to leak back into the ventricles. Regurgitation of the pulmonary valve can cause the right ventricle to work harder than the left side of the heart, leading to complications such as swelling in the body, particularly the legs, fatigue, shortness of breath, and a blue hue to the skin. Over time, regurgitation of the aortic valve can also cause fatigue and difficulty breathing.
Regurgitation and stenosis often affect the same valve. They are often caused by age-associated deterioration, an infectious disease such as endocarditis, rheumatic fever, or a birth defect. In the aortic valve, such a defect might include a congenital narrowness of the valve or a bicuspid aortic valve. A bicuspid aortic valve occurs when the SV has two flaps instead of three. Treatment for this condition often includes replacement of the aortic valve.