The oxygen poor blood enters the right atrium through the superior and inferior vena cava, is pumped through the tricuspid valve into the right ventricle, then it pumped through the pulmonary valve through the pulmonary arteries to the lungs.
The left atrium receives oxygen rich from the lungs and pumps it through the mitral, or bicuspid valve, to the left ventricle where it is pumped through the aortic valve to the aorta. The blood then goes through many other arteries then veins and goes back to the right atrium when oxygen poor.