Final answer:
The phase where about 20% of blood volume returns to the ventricles is the isovolumic ventricular relaxation phase, which is the early stage of ventricular diastole marked by the closing of the semilunar valves and the small reversal of blood flow indicated by the dicrotic notch.
Step-by-step explanation:
The phase in which 20% of the total volume returns to the ventricle during the cardiac cycle is known as the isovolumic ventricular relaxation phase, which is the initial phase of ventricular diastole. In this phase, the ventricles are relaxing and the pressure within them begins to drop. As pressure in the ventricles falls below the pressure in the pulmonary trunk and aorta, the semilunar valves (pulmonary and aortic) close to prevent backflow of blood into the heart. This small reversal of blood flow toward the heart is recorded as the dicrotic notch on blood pressure tracings.