During which phase does the ventricles fill with blood?

The correct choice focuses on the phase known as ventricular filling, which is a key aspect of the cardiac cycle. During this phase, the ventricles are in the process of filling with blood primarily from the atria. This occurs after the atria contract and push blood into the ventricles, allowing them to expand and prepare for the next contraction.

In ventricular filling, two main events take place: first, during diastole, the heart muscle relaxes, and the pressure in the ventricles falls, causing the atrioventricular (AV) valves (the tricuspid and mitral valves) to open. This allows blood to flow passively from the atria into the ventricles. The latter part of this phase may also involve atrial contraction, which helps to push any remaining blood into the ventricles to ensure they are optimally filled before the subsequent ventricular contraction occurs.

Understanding this phase is crucial for comprehending how the heart functions efficiently, ensuring that sufficient blood volume is present in the ventricles to supply the body's needs when the heart contracts. Thus, identifying ventricular filling as the correct answer highlights your grasp of cardiac physiology, particularly the processes that allow for effective blood circulation.