What are the semilunar valves?
There are four heart valves that regulate blood and act as doorways between the lower and upper chambers. Two of these valves pump blood out of the heart and are known as semilunar (SL) valves – the aortic valve (left semilunar valve) and the pulmonary valve (right semilunar valve). Because the leaflets (cusps) of both valves take the shape of half-moons, the valves are said to be “semilunar”.
What do the semilunar valves look like?
The SL valves are malleable, pocket-like structures that are made up of endocardium and fibre-reinforced connective tissue – this prevents the inversion of the valves. The aortic valve lies between the left ventricle and aorta. The pulmonary valve separates the pulmonary artery and right ventricle. Each valve is supported by a fibrous annulus (ring of tissue) and features cusps that meet tightly (coapt) when the valves close. Both the aortic valve and the pulmonary valve have three leaflets. Unlike the atrioventricular valves, though, they do not have chordae tendineae (heart strings) that attach to papillary muscles. Instead, they open and close due to the pressure of blood circulating within the heart between the atria and the ventricles.
What are the functions of the semilunar valves? How do they function?
The SL valves serve to enable blood flow from the ventricles to the aortic and pulmonary arteries when the valves open and to prevent blood from regurgitating into the ventricles when they close. The aortic valve regulates blood that enters the aorta from the left ventricle, while the pulmonary valve regulates blood entering the pulmonary artery from the right ventricle. Both valves open and close according to pressure differences between the atria and ventricles. When the ventricles relax, atrial pressure increases, forcing open the AV valves and allowing blood into the ventricles. When the ventricles contract, ventricular pressure increases, causing the AV valves to shut and the SL valves to open, which allows blood to pass through.