The main job of is to provide oxygen to the blood, and release the carbon dioxide left over from the spent blood, works with the cardiovascular system to make this happen. Blood enters the lungs through the pulmonary circulation and is dark colored, low in oxygen, and high in carbon dioxide. It is pumped by the right side of the heart into the pulmonary arteries, which lead to the lungs, the pulmonary arteries divide into smaller and smaller blood vessels, ending with pulmonary capillaries in the walls of the alveoli. The alveolar walls are ....