→ First of all, all the deoxygenated blood from the body with the HELP of superior and inferior vena cava came in heart. 

→ After it, it came in Right atrium and then Right ventricle.

→ Contraction of auricles derives most of the blood in into the ventricle. At the END of the auricular systole, auricles start relaxing and the contraction of the ventricles simultaneously.

→ As the contraction of the ventricles begins, the pressure of the blood in the auricles rises. With this pressure, the flaps of the auricular ventricular (A-V) valves are PUSHES up and meet to close the A-V apertures. This tight CLOSURE of the A-V valves at the start of the ventricular contraction produces the first heart beat, which is "Lub".

→ Now the ventricles start relaxing. The semilunar VALVE close as they get filled the blood trying to return and meet together, closing the outlets of the great arteries. 

→ The closing of the semilunar valves at the start of diastole produces the second heart sound "Dup".

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