During which phase is the heart filling with blood from the atria?

The correct answer is associated with the phase known as diastole. During diastole, the heart muscles relax, allowing the heart chambers, particularly the ventricles, to fill with blood. This phase is crucial for preparing the heart to pump blood effectively during the subsequent contraction phases.

In diastole, blood flows from the atria into the ventricles due to the pressure gradient created by the relaxation of the ventricles, leading to passive filling. Additionally, atrial systole, which occurs right after diastole, does further contribute to ventricular filling by actively pushing blood from the atria into the ventricles. However, the question specifically asks when filling occurs generally, which encompasses both passive filling during diastole and active filling influenced by atrial contraction.

Other phases listed do not align with this filling process. For instance, during ventricular systole, the ventricles contract and pump blood out to the lungs and the rest of the body, making it a period of ejection rather than filling. Isovolumetric contraction is a stage of the cardiac cycle right before ventricular systole, where all heart valves are closed, and no blood enters or leaves the ventricles. Thus, diastole is the phase that most accurately