The blood (without oxygen) enters the right atrium of the heart through the inferior and superior vena cavas. From the right atrium, it flows to the right ventricle through the tricuspid valve. Then, the right ventricule squeezes when it's full, closing the tricuspid valve and opening the pulmonary valve. The blood then flows through the pulmonary arteries, entering the lungs, where it gets oxygen. The blood will then leave the lungs through the pulmonary veins, entering the left atrium. From the left atrium it flows through the mitral valve, going to the left ventricle. It leaves the left ventricle and leaves the heart through the aortic valve, flowing in the aorta to reach the rest of the body.