semilunar valves constitute the
second set of heart valves and are named for each of the three
crescent-shaped cusps that make up each valve. Each of the two
large arteries receiving blood from the ventricles has a
semilunar valve at its proximal end that prevents the backflow of
blood into the ventricle during diastole. The valves are
supported by an annulus ring.As the ventricles contract the semilunar valves are forced to open and the cusps are forced against the artery wall. At the end of systole blood from the arteries fills the cusps and forces them toward the ventricles. The free edges of the cusps meet or coapt to form a barrier preventing the backflow of blood into the ventricles.
The right and left 
coronary arteries arise just above
the cusps of aortic semilunar valve. A dilation of the aorta
occurs adjacent to each cusp each forming the Sinuses of
Valsalva.