Deeper into the lungs, each bronchus subdivides into secondary and tertiary bronchi, and then into smaller airways called bronchioles. In contrast to the bronchi, the bronchioles are not made of rigid cartilage and are therefore subject to constriction and obstruction, as which occurs during a COPD exacerbation. The bronchioles end in air sacs called alveoli. The alveoli are the site of gas exchange within the lungs.
Alveoli are tiny, microscopic structures that are bunched together in grape-like clusters to form alveolar sacs. On the surface of the alveoli are networks of capillaries (tiny blood vessels) that carry blood from the veins of other parts of the body. It is here where gas exchange occurs -- carbon dioxide from the blood is exchanged for oxygen from the alveoli. After the oxygenated blood leaves the alveoli, it travels to the heart, located between the two lungs, where it is pumped out to the rest of the body. The carbon dioxide is then expelled from your body each time you exhale.