Final answer:
Stage 4: Rapid Filling of the cardiac cycle is the phase where the ventricles fill with blood while the atria are relaxed. This occurs during ventricular diastole, following atrial systole and preceding isovolumetric contraction.
Step-by-step explanation:
The stage of the cardiac cycle that involves rapid filling of the ventricles while the atria are relaxed is Stage 4: Rapid Filling. During this phase, the ventricles are being filled with blood, which primarily happens during diastole, the period of relaxation for the chambers of the heart. After the atria contract and move blood into the ventricles (atrial systole), there comes a point when the atria enter a relaxed state known as atrial diastole. It is during ventricular diastole that the ventricles fill rapidly with blood, followed by slower filling as the cycle approaches the next atrial systole.
The term used to describe ventricular contraction when no blood is being ejected is isovolumetric contraction. This is an event within the cardiac cycle where the ventricular muscles contract, increasing the pressure inside the ventricles, but the volume of blood within does not change because the semilunar valves (pulmonary and aortic) have not yet opened to allow blood to be ejected.