As blood flows through the body, it gives oxygen to the tissues and receives carbon dioxide plus other wastes from those tissues. This deoxygenated blood enters the veins and finally arrives at two major veins, the inferior and superior vena cava which finally empty at the right atrium of the heart. Blood is then pumped through the tricuspid valve into the right ventricle, and from the right ventricle, its pumped through the semi-lunar valve into the pulmonary artery. Blood then goes to the lungs via the pulmonary arteries (one for each lung) and is reoxygenated and then flows into the pulmonary veins, which empty at the left atrium, which pumps it through the bicuspid valve (mitral valve), into the left ventricle, which then pumps it through the aortic semi-lunar valve into the aorta, which distributes this oxygenated blood all over the tissues to deliver oxygen. And the cycle is repeated.