Deoxygenated blood is transported back to the heart by the superior and inferior vena cava, it enters the right atrium, and it passes the opened tricuspid atrioventricular valve to the right ventricle.
The cardiomyocytes of the right ventricle contract and push the blood through the pulmonary semilunar valves to the pulmonary arteries. The blood flows through the pulmonary system, where the gases are exchanged, and the blood is oxygenated.
Then the blood returns to the heart through the pulmonary veins to the left atrium and past the bicuspid atrioventricular valve to the left ventricle.
When the left ventricle contracts, the blood is pushed through the aortic semilunar valves into the aorta where it enters the systemic circulation.