Final answer:
During the isovolumic ventricular relaxation phase of the cardiac cycle, both atrioventricular and semilunar valves are closed and pressure decreases in the ventricles without a change in volume, marking the initial stage of ventricular diastole.
Step-by-step explanation:
The phase of the cardiac cycle when all four valves are closed and the pressure in the ventricles is decreasing is known as the isovolumic ventricular relaxation phase. This phase occurs during the early stage of ventricular diastole. As the ventricular muscles relax, the pressure in the ventricles begins to fall, dropping below that in the pulmonary trunk and aorta. The backflow of blood toward the heart results in the closure of the semilunar valves to prevent regurgitation into the ventricles. Since the atrioventricular valves are still closed at this point, the volume of blood in the ventricles doesn't change, resulting in isovolumetric conditions. This is indicated by the dicrotic notch on a blood pressure tracing.