Heart and Lungs
The heart and lungs are the primary contents of the thorax. They are interconnected with very large blood vessels. The heart sends oxygen-poor blood through the pulmonary arteries to the lungs, which oxygenate it and return it to the heart through the pulmonary veins. The pulmonary arteries arise from one large pulmonary trunk, and then begin branching exponentially once they enter the lungs in order to reach the functional respiratory units and pick up oxygen. The smallest pulmonary veins then take the oxygenated blood backwards through the lungs and empty into the back of the heart through four larger pulmonary veins. The oxygen-rich blood is then pumped by the heart out into the body through the aorta. Deoxygenated blood from body tissues returns to the heart through the superior and inferior vena cava and the cardiac cycle repeats continuously. The pulmonary veins and arteries are the only case where arteries carry deoxygenated blood and veins carry blood that has been oxygenated.