To reason about COPD you first need a clear picture of normal gas exchange. In healthy lungs, air moves through progressively smaller airways to reach the alveoli, thin-walled sacs wrapped in pulmonary capillaries where oxygen diffuses into blood and carbon dioxide diffuses out. This exchange depends on three things staying intact: open airways, elastic tissue that recoils to push air back out, and a large healthy surface area where blood and gas meet. COPD damages all three, which is why gas exchange fails in more than one way. COPD is really an umbrella term for two overlapping processes that usually coexist. Chronic bronchitis is defined clinically by a productive cough for at least three months in two consecutive years. Here the problem is the airways themselves. Chronic exposure to tobacco smoke or other irritants triggers persistent inflammation, the mucus-producing glands enlarge, and the airway walls thicken. The result is narrowed, mucus-clogged airways that obstruct airflow and trap bacteria, which is why these patients cough, produce sputum, and are prone to infection. Emphysema, by contrast, is a problem of the alveoli...