The 'lub- dub' is the sound of your heart valves opening and closing. (S1 or 'lub' ) is caused by closing of the mitral and tricuspid valve
as the ventricles begin to contract and pump blood into the aorta
. The second sound (S2 or 'dub') is caused when the ventricles begin to relax and allow the aortic and the pulmonary valves to close.