Explanation:
Semilunar valves are the heart valves that are NOT actively closed by the contraction of muscular structures. These valves include the aortic valve and the pulmonic valve.

Function of Semilunar Valves:
Semilunar valves are responsible for preventing the backflow of blood from the aorta and pulmonary artery into the ventricles during diastole (relaxation phase of the cardiac cycle). These valves have three semilunar cusps that are shaped like half-moons, hence the name "semilunar."

Mechanism of Semilunar Valve Closure:
The closure of semilunar valves is primarily passive and does not involve the contraction of muscular structures. Instead, it is based on pressure differences within the heart chambers. When the ventricles relax during diastole, the pressure within the ventricles decreases. At the same time, the pressure in the aorta and pulmonary artery remains higher due to the elastic recoil of these vessels. This pressure difference causes the semilunar valves to close, preventing the backflow of blood into the ventricles.

Preventing Backflow:
The semilunar valves open when the pressure in the ventricles exceeds the pressure in the aorta or pulmonary artery during ventricular systole (contraction phase of the cardiac cycle). This allows blood to be ejected from the ventricles into the respective arteries. However, as soon as ventricular systole ends and the ventricles relax, the pressure in the arteries becomes higher than the pressure in the ventricles. This pressure difference causes the semilunar valves to close, effectively preventing backflow of blood into the ventricles.

Summary:
Semilunar valves, including the aortic and pulmonic valves, are not actively closed by the contraction of muscular structures. They function to prevent the backflow of blood from the aorta and pulmonary artery into the ventricles. The closure of these valves is primarily passive and occurs due to pressure differences within the heart chambers.