Which of the following statements is correct for cardiac cycle?

Options

A. The ventricular systole is stimulated by the SA node.
B. The first heart sound coincides with the closure of semilunar valves and relaxation of ventricles.
C. The first heart sound coincides with opening of AV valves
D. The fall in ventricular pressure causes the closure of semilunar valve to prevent the back-flow of blood into ventricles.

Answer

To determine which statement is correct regarding the cardiac cycle, we will analyze each option provided in the question step by step.

### Step-by-Step Solution:

1. **Understanding the Cardiac Cycle**: The cardiac cycle consists of a sequence of events that occur during one heartbeat, which lasts approximately 0.8 seconds. It includes both systole (contraction) and diastole (relaxation) phases of the heart.

2. **Evaluating Option 1**:
- **Statement**: "The ventricular systole is stimulated by the acenoad."
- **Analysis**: This statement is incorrect. The sinoatrial (SA) node, often referred to as the pacemaker of the heart, initiates the action potential that causes atrial contraction (systole). Ventricular systole is triggered by the conduction of this impulse through the atrioventricular (AV) node and the bundle of His, not directly by the SA node.

3. **Evaluating Option 2**:
- **Statement**: "The first heart sound coincides with the closure of the semilunar valves and relaxation of the ventricles."
- **Analysis**: This statement is also incorrect. The first heart sound (S1) is produced by the closure of the atrioventricular (AV) valves (tricuspid and mitral valves), not the semilunar valves. The closure of the semilunar valves occurs after ventricular contraction, leading to the second heart sound (S2).

4. **Evaluating Option 3**:
- **Statement**: "The first heart sound coincides with the opening of the AV valve."
- **Analysis**: This statement is incorrect. The first heart sound occurs when the AV valves close, marking the beginning of ventricular contraction. The opening of the AV valves happens during diastole, which does not coincide with the first heart sound.

5. **Evaluating Option 4**:
- **Statement**: "The fall in ventricular pressure causes the closure of the semilunar valve to prevent the backflow of blood into the ventricles."
- **Analysis**: This statement is correct. After ventricular contraction, as the ventricles relax, the pressure within them decreases. This drop in pressure causes the semilunar valves (pulmonary and aortic valves) to close, preventing the backflow of blood into the ventricles.

### Conclusion:
The correct answer is **Option 4**: "The fall in ventricular pressure causes the closure of the semilunar valve to prevent the backflow of blood into the ventricles."