All blood is returned to the heart via the superior and inferior vena cava, the two largest veins in the body. The superior vena cava drains blood from the head and neck into the right atrium of the heart. The inferior vena cava drains the remainder of the body, this blood also flows into the right atrium of the heart. Blood leaves the right atrium, passes through the tricuspid valve and enters the right ventricle. From the right ventricle it is passed to the lungs via pulmonary arteries and returns to the left atrium of the heart from the lungs via pulmonary veins. Blood leaves the left atrium by passing through the bicuspir/mitral valve into the left ventricle of the heart. Blood now enters the aorta and is circulated throughout the body.
From the vena cava, the deoxygenated blood is returned to the right atrium. Here, it flows down to the right ventricle. It is then pumped to the pulmonary arteries which lead to both lungs. It enters capillaries in the lungs and carbon dioxide is exchanged for oxygen. Pulmonary veins transport blood from the lungs to the left atrium. The blood flows down to the left ventricle. Here, it is pumped out to the aorta, the body's largest artery, which has branches to all parts of the body. These become smaller arteries, arterioles and finally capillaries. This blood is oxygenated and must be transported to the body cells. Eventually, the cells absorb the oxygen and carbon dioxide is sent to the circulating blood. The inferior and superior vena cava again return deoxygenated blood to the right atrium and the cycle repeats.