It enters the superior and inferior vena cava and dumps into the right atrium and through to the right ventricle. From the right ventricle, it enters the pulmonary artery (the only artery to carry deoxygenated blood) and to the lungs. Gas exchange happens in the alveolar capillaries and exits to be into the pulmonary vein (the only vein to carry oxygenated blood). It enters from the pulmonary vein into the left atrium and down into the left ventricle. From here, it is ready to go into the body and it's pumped through the aorta.
Blood enters the heart through two large veins, the inferior and superior vena cava, emptying oxygen-poor blood from the body into the right atrium. The pulmonary vein empties oxygen-rich blood, from the lungs into the left atrium.
