Final answer:
As the atria relax during late ventricular diastole in the cardiac cycle, the pressure in the ventricles falls and leads to the reopening of the tricuspid and mitral valves.
Step-by-step explanation:
During the final stage of the cardiac cycle, specifically during the phase known as late ventricular diastole, the atria relax and the ventricles are filled with blood returning from the atria. As the ventricular muscle relaxes, the pressure inside the ventricles falls below that of the atria. This pressure differential causes the atrioventricular valves, namely the tricuspid and mitral valves, to reopen allowing blood to fill the ventricles. The semilunar valves, which include the pulmonary and aortic valves, remain closed during this phase to prevent backflow into the ventricles from the pulmonary artery and aorta.