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 the statements provided based on our understanding of the cardiac cycle.

### Step-by-Step Solution:

1. **Understanding the Cardiac Cycle**:
- The cardiac cycle refers to the sequence of events that occur during one heartbeat, including the contraction and relaxation of the heart chambers (atria and ventricles).
- It consists of two main phases: diastole (relaxation) and systole (contraction).

2. **Analyzing the Valves**:
- The heart has four main valves: the tricuspid valve (between the right atrium and right ventricle), the bicuspid (mitral) valve (between the left atrium and left ventricle), and the semilunar valves (the aortic valve and the pulmonary valve).
- During the cardiac cycle, the valves open and close in response to pressure changes in the heart chambers.

3. **Evaluating the Statements**:
- **Statement 1**: "There is no time in the cardiac cycle when both the mitral and aortic valves are open."
- This statement is correct. The mitral valve and aortic valve cannot be open simultaneously because when the ventricles contract (systole), the aortic valve opens while the mitral valve closes to prevent backflow into the atria.

- **Statement 2**: "In the left atrium, there is more pressure than the left ventricle during the ventricular ejection period."
- This statement is incorrect. During ventricular ejection, the pressure in the ventricles is higher than in the atria, causing the aortic valve to open and blood to flow into the aorta.

- **Statement 3**: "The ventricles fill the most during diastole when the atria contract."
- This statement is misleading. While the atrial contraction does contribute to ventricular filling, the ventricles fill primarily during the passive phase of diastole when blood flows from the atria into the ventricles without atrial contraction.

- **Statement 4**: "The second heart sound is associated with the left atrioventricular valve (mitral valve) closure."
- This statement is incorrect. The second heart sound (dub) is associated with the closure of the semilunar valves (aortic and pulmonary valves), not the mitral valve.

4. **Conclusion**:
- The only correct statement about the cardiac cycle is the first one: "There is no time in the cardiac cycle when both the mitral and aortic valves are open."