The Semilunar Valves
The aortic and pulmonary valves are similar to one another in structure but are different from the atrioventricular valves. Their leaflets of delicate tissue are shaped like crescents, thus the name "semilunar" (half-moon) valves. The pulmonary valve is at the opening from the right ventricle to the pulmonary artery, and the aortic valve is at the opening from the left ventricle to the aorta.
In contrast to the atrioventricular valves, the semilunar valves have no chordae tendineae and are structurally simpler. The aortic valve leaflets are thicker and more opaque than the pulmonary valve leaflets.
Blood flowing from the ventricles pushes the leaflets of the semilunar valves toward the artery walls contraction, allowing blood to pass through freely. When the blood flow from the ventricles decreases, the pressure in the ventricles falls. The pr in the artery soon becomes higher than that in the ventricles. When this happens, the blood under higher pr in the arteries presses the leaflets downward, and the valves snap closed
One-Way Blood Flow
The valves are designed to allow b to pass in only one direction, valves do not actively open and c that is, they do not automatically open when blood is approaching. Inst they function like a gate that o| only when it is pushed and is bui such a way that it will open in only direction. The valves open and c in response to the naturally occur pressure differences that build within the heart's chambers during systolic and diastolic portions of e cardiac cycle.
For example, the aortic valve op to allow blood to eject from the ventricle into the aorta, because during systole (contraction) the press in the left ventricle is higher than t in the aorta. This pressure difference forces open the leaflets and allows blood to flow through the aortic valve.
During diastole, when the left ventricle relaxes, the pressure in the ventricle becomes low again while the pressure in the aorta remains high. T valve is pushed closed by the pressure and blood is prevented from leaking back.