In reference to a double-loop circulatory system, blood travels through two different pathways in the body. In humans, the vena cava which are the largest veins in the body, return deoxygenated blood to the right side of the heart. It travels from the right atrium, to the right ventricle, where it is pumped out of the heart and travels to the lungs via the pulmonary arteries. Inside the lung capillaries, carbon dioxide is exchanged for oxygen and the blood becomes oxygenated. This is one part of the loop. The second part is when oxygenated blood travels from the lungs to the left side of the heart, via the pulmonary veins. The left atrium pumps the blood to the left ventricle, which pumps it to all parts of the body through the aorta. This is the largest artery in the body and it has branches to all parts of the body. Eventually, this blood becomes deoxygenated and the pathway continues once again.