Which of the following events occur during late ventricular diastole?
A. The atria are relaxed, the ventricles are filling passively, the atrioventricular valves are open.
B. The ventricles are starting to contract, the atrioventricular valves are closed, the semilunar valves are open.
C. The atria contract, the ventricles are relaxed, the atrioventricular valves are open.
D. The atria are relaxed, the ventricles are starting to relax, the atrioventricular valves are closed, the semilunar valves are closed.
The Correct Answer and Explanation is :
The correct answer is A: The atria are relaxed, the ventricles are filling passively, and the atrioventricular valves are open.
Explanation:
Late ventricular diastole is a phase in the cardiac cycle when the heart is in the process of filling with blood, preparing for the next contraction. It consists of the following events:
- Ventricular Relaxation: During late ventricular diastole, the ventricles are relaxed. This allows them to expand and accommodate blood from the atria. Blood flows from the atria into the ventricles due to the pressure difference between the two chambers.
- Atrioventricular (AV) Valves Open: The AV valves (the mitral valve on the left and the tricuspid valve on the right) are open during this phase. The pressure in the atria is higher than in the ventricles, which causes these valves to open and allows blood to passively flow into the ventricles.
- Atria Relaxed: The atria are not contracting during late ventricular diastole. Instead, they are in a relaxed state after the previous contraction phase, and they are filling with blood that flows back from the systemic and pulmonary circuits.
- Ventricular Filling: The filling of the ventricles occurs passively in late diastole, meaning no active contraction of the atria is involved. The ventricles receive about 70-80% of their total blood volume during this phase. The final 20-30% of ventricular filling occurs during atrial contraction (the atrial systole), but that occurs just after the late diastolic phase.
Why Other Answers Are Incorrect:
- B: This describes events of ventricular systole, when the ventricles contract and the semilunar valves open. The AV valves are closed during this phase.
- C: This describes the atrial systole phase, where the atria contract, but it doesn’t match the events of late diastole when the ventricles are relaxed and filling passively.
- D: In late diastole, the AV valves are open and the ventricles are relaxed, so this description doesn’t align with the correct events of late diastole. Additionally, the semilunar valves are closed during late diastole.