Final answer:
Early diastole marks the start of the ventricles relaxing, with pressure falling within the ventricles--a phase called isovolumic ventricular relaxation. During this time, the semilunar valves close, preventing backflow into the heart, and no change in ventricular volume happens as the atrioventricular valves remain shut.
Step-by-step explanation:
During early diastole, the heart's ventricular muscles relax causing a decrease in pressure within the ventricles. This phase is characterized by the isovolumic ventricular relaxation phase, where pressure falls below that of the major arteries, leading to a small backflow that closes the semilunar valves and produces a dicrotic notch on blood pressure tracings. Importantly, during early diastole, the atrioventricular valves are still closed, so ventricular volume remains unchanged, distinguishing this isovolumetric phase.