This is a false-colour arteriogram of the pulmonary arteries in a healthy human lung. The main pulmonary artery divides into thick left and right branches, one to each lung, carrying large quantities of blood. Inside the lungs the arteries divide repeatedly to form a network of fine capillaries, maximizing the area of surface contact (an astonishing 70 square meters) between the blood and lung tissues for the exchange of gases. Oxygen from inhaled air enters the bloodstream, while waste carbon dioxide is released and exhaled.