Semilunar valves close when

Options

A. Ventricular pressure rises
B. Atrial pressure rises
C. Ventricular pressure falls
D. Atrial pressure falls

Answer

### Step-by-Step Solution:

1. **Understanding the Role of Semilunar Valves**:
- The semilunar valves are located between the ventricles and the major arteries (the aorta and the pulmonary artery). Their primary function is to prevent the backflow of blood from the arteries into the ventricles.

2. **Pressure Changes During the Cardiac Cycle**:
- The heart goes through a cycle of contraction (systole) and relaxation (diastole). During systole, the ventricles contract, and blood is pumped into the arteries.

3. **Opening of Semilunar Valves**:
- When the ventricles contract, the pressure within the ventricles rises. This pressure must exceed the pressure in the arteries for the semilunar valves to open, allowing blood to flow into the aorta and pulmonary artery.

4. **Closing of Semilunar Valves**:
- After the ventricles have contracted and the blood has been ejected into the arteries, the ventricles begin to relax. As the ventricles relax, the pressure within them decreases.
- Meanwhile, the pressure in the arteries remains higher than that in the ventricles right after the contraction. However, as the ventricles continue to relax, the pressure in the arteries will eventually drop below the pressure in the ventricles.

5. **Atrial Pressure Increase**:
- As the ventricles relax, the atria begin to fill with blood, causing the pressure in the atria to rise. This increase in atrial pressure contributes to the closure of the semilunar valves.

6. **Conclusion**:
- The semilunar valves close primarily when the atrial pressure rises, which occurs after the ventricles have contracted and are in the process of relaxing. This prevents backflow of blood from the arteries into the ventricles.

### Final Answer:
The semilunar valves close when the atrial pressure rises.