Answer:
When the ventricles receive blood from atria, both ventricles contract simultaneously and the blood is pumped to pulmonary arteries and aorta. The tricuspid and bicuspid valves close, and LUBB sound is made. Ventricular systole end, and ventricles relax at the same time semilunar valves at the base of pulmonary artery and aorta close simultaneously, and DUBB sound is made.
