To understand COPD, start with what healthy lungs do. With each breath, air travels down the trachea and bronchi into millions of tiny air sacs called alveoli. The alveoli sit against a fine mesh of capillaries, and this is where gas exchange happens. Oxygen diffuses from the alveoli into the blood, and carbon dioxide, the waste product of metabolism, diffuses out of the blood into the alveoli to be exhaled. This exchange works well only when three things are intact: open airways that let air flow freely, healthy stretchy alveoli with plenty of surface area, and good blood flow matched to the areas being ventilated. COPD damages all three. COPD is usually a blend of two overlapping problems: chronic bronchitis and emphysema. In chronic bronchitis, the airways themselves are chronically inflamed. The lining swells, and the mucus-producing glands enlarge and pump out thick, excessive mucus. That mucus narrows the airway and becomes a breeding ground for infection, which is why these patients cough daily and are prone to recurrent chest infections. In emphysema, the problem is destruction of the alveolar...