Opening of the AV valves allows the ventricles to:

• A. Fill with blood
• B. Eject blood into the arteries
• C. Return blood to the atria
• D. Oxygenate systemic blood

Answer: (A)

Opening of the atrioventricular valves lets the ventricles fill with blood. When the atria have higher pressure in diastole, the mitral and tricuspid valves open, allowing blood to flow from the atria into the ventricles (mostly passively, with a small contribution from the atrial contraction, the atrial kick). This filling prepares the ventricles for the next contraction. The other options describe later steps: ejection into the arteries happens after the ventricles contract and the semilunar valves open; returning blood to the atria would involve backflow during contraction; oxygenating systemic blood occurs in the lungs and is not driven by AV valve opening.