During passive ventricular filling, which valves open?

• A. AV valves
• B. Semilunar valves
• C. Both AV and semilunar valves
• D. No valves open

Answer: (A)

During passive ventricular filling, the ventricles are relaxed and have low pressure. Blood returning from the veins fills the atria and then flows into the ventricles because the pressure in the atria is momentarily higher than in the ventricles. This causes the atrioventricular valves (the mitral and tricuspid valves) to open, allowing a large amount of blood to fill the ventricles passively. The semilunar valves (the aortic and pulmonary valves) remain closed at this stage because ventricular pressure is not yet high enough to push blood into the arteries. Later, atrial contraction adds a final amount of blood, but the initial filling is passive via open AV valves.