What occurs at the beginning of the cardiac cycle?

What occurs at the beginning of the cardiac cycle?

• A. All four chambers are at rest, blood returns to the right and left atrium, and passive filling of ventricles occurs.
• B. Ventricles actively eject blood immediately.
• C. Atria are contracting to push blood into ventricles.
• D. SL valves are open.

Answer: (A)

At the start of the cardiac cycle the heart is in diastole: all chambers are relaxed and ventricles are filling. Blood returns to the atria from the body and lungs, and it flows passively into the ventricles through the open atrioventricular valves. The semilunar valves stay closed to prevent backflow into the ventricles. This phase creates the pressure gradient that allows rapid passive filling of the ventricles before any atrial contraction adds the final preload. That’s why the description of all four chambers at rest with passive ventricular filling best fits the beginning of the cycle. The other ideas describe later events: ventricular ejection happens during systole, atrial contraction occurs after passive filling to push remaining blood into the ventricles, and the semilunar valves are open only during ventricular ejection, not at the start.

At the start of the cardiac cycle the heart is in diastole: all chambers are relaxed and ventricles are filling. Blood returns to the atria from the body and lungs, and it flows passively into the ventricles through the open atrioventricular valves. The semilunar valves stay closed to prevent backflow into the ventricles. This phase creates the pressure gradient that allows rapid passive filling of the ventricles before any atrial contraction adds the final preload.

That’s why the description of all four chambers at rest with passive ventricular filling best fits the beginning of the cycle. The other ideas describe later events: ventricular ejection happens during systole, atrial contraction occurs after passive filling to push remaining blood into the ventricles, and the semilunar valves are open only during ventricular ejection, not at the start.