During which physiological process do the ventricles fill with blood?

The ventricles fill with blood during diastole, which is the phase of the cardiac cycle when the heart muscle relaxes. During this relaxation phase, the pressure within the ventricles decreases, allowing blood to flow from the atria into the ventricles. This filling process is essential for ensuring that when the heart contracts during the subsequent phase (systole), it can pump an adequate volume of blood out to the lungs and the rest of the body.

In contrast, during systole, the ventricles contract to pump blood out of the heart, and during exhalation, there is no direct relation to the filling of the ventricles, as it primarily pertains to the respiratory system. Contraction refers specifically to the phase of systole where the heart is actively working to pump blood, not the filling process.