During which phase does the heart expand to fill with blood?

The heart expands to fill with blood during diastole. This phase is essential for the heart's function as it allows the chambers, specifically the atria and ventricles, to relax and increase in volume, creating a low-pressure environment that facilitates the intake of blood returning to the heart. During diastole, the atrioventricular (AV) valves open, allowing blood to flow from the atria into the ventricles.

This filling process is crucial for maintaining adequate blood circulation throughout the body, as it sets the stage for the subsequent contraction phase, where blood is pumped out of the heart. In contrast, systole refers to the phase when the heart muscle contracts to push blood out to the lungs and the rest of the body, which does not involve expansion for filling. The terms febrile and ventricular contraction are not related to the filling process of the heart and pertain to different aspects of physiology, with febrile often relating to fever and ventricular contraction indicating active pumping rather than filling.