Final answer:
During the isovolumic ventricular relaxation phase of ventricular diastole, the lowest pressure is reached when the ventricle is filling. In this phase, the pressure in the ventricles drops below the pressure in the pulmonary trunk and aorta, causing blood to flow back towards the heart.
Step-by-step explanation:
The lowest pressure during cardiac relaxation, when the ventricle is filling, occurs during the isovolumic ventricular relaxation phase of ventricular diastole. In this phase, as the ventricular muscle relaxes, the pressure in the ventricles drops below the pressure in the pulmonary trunk and aorta, causing blood to flow back towards the heart. This phase is also characterized by the closure of the semilunar valves to prevent backflow into the heart.