Blood enters the heart through two large veins, the inferior and superior vena cava, emptying into the right atrium. It flows from the right atrium through the tricuspid valve into the right ventricle. The blood then leaves the right ventricle and heart through the pulmonic valve and flows into the pulmonary artery and to the lungs. Oxygenated blood returns from the lungs to the heart via the pulmonary vein into the left atrium. From the left atrium, blood flows through the mitral valve to the left ventricle. From the left ventricle blood leaves the heart through the aortic valve and flows into the aorta and to the body!!

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"