Which of the following statements about the cardiac cycle is CORRECT?

Options

A. At no time during the cardiac cycle are the mitral and aortic valves both open.
B. The pressure in the left atrium exceeds that of the left ventricle throughout the ventricular ejection period.
C. Most filling of the ventricle in diastole in a resting individual occurs with atrial contraction.
D. The second heart sound is most closely associated with the closure of the left atrioventricular (mitral) valve.

Answer

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

### Step-by-Step Solution:

1. **Understanding the Cardiac Cycle**:
- The cardiac cycle consists of a series of events that occur during one heartbeat, including atrial contraction, ventricular contraction, and relaxation phases.

2. **Analyzing the First Statement**:
- The first statement claims that "at no time during the cardiac cycle are the mitral and the aortic valves both open."
- During atrial contraction, the mitral valve opens to allow blood flow from the left atrium to the left ventricle, while the aortic valve is closed.
- During ventricular contraction, the aortic valve opens to allow blood to flow from the left ventricle into the aorta, while the mitral valve is closed.
- Therefore, this statement is **correct**.

3. **Analyzing the Second Statement**:
- The second statement states that "the pressure in the left atrium exceeds that of the left ventricle throughout the ventricular ejection period."
- During ventricular ejection, the pressure in the left ventricle is significantly higher than that in the left atrium due to ventricular contraction. Thus, this statement is **incorrect**.

4. **Analyzing the Third Statement**:
- The third statement claims that "most of the filling of the ventricle in diastole in a resting individual occurs with atrial contraction."
- In reality, about 70-90% of ventricular filling occurs passively before atrial contraction, with only about 10-30% occurring during atrial contraction. Therefore, this statement is **incorrect**.

5. **Analyzing the Fourth Statement**:
- The fourth statement mentions that "the second heart sound is most closely associated with the closure of the atrioventricular valve which is the mitral valve."
- The second heart sound (dub) is actually associated with the closure of the aortic and pulmonary valves, not the mitral valve. Thus, this statement is **incorrect**.

### Conclusion:
The only correct statement regarding the cardiac cycle is the first one: "At no time during the cardiac cycle are the mitral and the aortic valves both open."

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