During what physiological condition is the right ventricle primarily filling with blood?

The right ventricle primarily fills with blood during diastole, which is the phase of the cardiac cycle when the heart muscles relax. During this time, the ventricles are filled with blood returning from the body (in the case of the right ventricle) through the superior and inferior vena cavae. Diastole consists of two parts: early diastole, when the atrioventricular valves (the tricuspid valve for the right ventricle) open and blood passively flows into the ventricles; and late diastole, when the atria contract to push additional blood into the ventricles.

In contrast, during systole, the ventricles contract to pump blood out of the heart, leaving little to no time for the filling. Atrial contraction, while contributing to ventricular filling, occurs at the end of diastole, primarily aiding in ensuring that the ventricles are full before they contract. Cardiac arrest occurs when the heart stops beating effectively, thus not allowing for any phase of filling to take place. Therefore, the correct answer clearly highlights the specific phase in the cardiac cycle dedicated to ventricular filling.