When does ventricular filling primarily occur?

Ventricular filling primarily occurs during mid to late diastole. During this phase of the cardiac cycle, the heart is relaxed, and the atrioventricular valves (the mitral and tricuspid valves) are open. This allows blood to flow passively from the atria into the ventricles. As the atria contract towards the end of diastole, they push additional blood into the ventricles, ensuring that they are filled adequately before the next contraction.

While early diastole does involve some filling as blood enters the ventricles due to the lower pressure in the ventricles compared to the atria, the majority of ventricular filling occurs during mid to late diastole when the pressures and volume changes more significantly favor complete filling. Systole is when the ventricles contract to pump blood out of the heart, and isovolumetric contraction refers to the brief phase where the ventricles contract without any volume change in the heart chambers, as the valves are closed. Therefore, mid to late diastole is the key period for effective ventricular filling due to both passive and active mechanisms.