Explanation

During working of heart, two sounds are produced lubb and dup. First sound ($i.e.,\$lubb) is produced, when auriculoventricular (tricuspid and bicuspid) valves are closed or at the end of diastole. The second sound ($i.e.,$ dup) is produced when semilunar valves at the base of dorsal aorta are closed or at the end of systole.