Which phase of the cardiac cycle is characterized by ventricular filling?

The phase of the cardiac cycle characterized by ventricular filling is indeed Phase One. During this phase, blood flows from the atria into the ventricles, facilitated by the relaxation of the heart muscle and the opening of the atrioventricular valves (the mitral and tricuspid valves). This filling process occurs primarily during diastole when the heart is relaxed, allowing the chambers to expand and accept blood.

In this phase, pressure decreases in the ventricles, which creates a pressure gradient that encourages blood flow from the higher-pressure atria. The majority of the ventricular filling occurs passively when the atria are full and contracts towards the end of this phase, contributing to the final filling of the ventricles just before contraction begins in the subsequent phase. Understanding this phase is crucial as it sets the foundation for effective cardiac output during the following phases of the cardiac cycle.