Blood that has travelled through the body enters the right atrium via the superior and inferior caval vein. It then travels to the right ventricle, passing the tricuspid valve. It is then pumped into the pulmonary artery, towards the lungs. In the lungs, oxygen is bound to the blood; it becomes oxygen-rich. Subsequently, the oxygen-rich blood enters the left atrium. Passing the Mitral valve, the blood enters the left ventricle. The left ventricle pumps the blood into the aorta, upon which it travels to all the organs and tissues in the body, providing them from oxygen. Once it has done so, it travels back to the right atrium.