The aortic semilunar valve prevents the backflow of blood when the left ventricle of the heart is returning to the relaxed state after pumping. The aorta is the largest artery in the body, and despite its size, is under high pressure. The pumping of the left ventricle, the largest chamber of the heart, exceeds the pressure in the aorta forcing blood in, but when it stops, aortic pressure is greater.
The aortic semilunar valve has three cusps, and it is constructed from tough fibrous proteins. It works automatically and is constructed to allow blood flow in one direction but is pushed shut when it begins flowing in the other, according to Boundless. The aorta is the first, largest artery that carries the oxygenated blood that travels everywhere in the body. The left ventricle produces enough pressure to force blood along the aorta, but in order to be refilled from the left atrium, which receives oxygenated blood from the lungs, its internal pressure must drop. Without the aortic semilunar valve, much of the blood the left ventricle pumped into the aorta would flow back into the ventricle when it relaxes.
The aortic semilunar valve is similar to the pulmonary valve, which also has three cusps and serves the same function, except that it serves the right ventricle and pulmonary artery.