Chronic bronchitis is defined by excessive mucus production with airway obstruction and notable hyperplasia of mucus-producing glands. Damage to the endothelium impairs the mucociliary response that clears bacteria and mucus. Inflammation and secretions provide the obstructive component of chronic bronchitis. In contrast to emphysema, chronic bronchitis is associated with a relatively undamaged pulmonary capillary bed. Emphysema is present to a variable degree but usually is centrilobular rather than panlobular. The body responds by decreasing ventilation and increasing cardiac output. This V/Q mismatch results in rapid circulation in a poorly ventilated lung, leading to hypoxemia and polycythemia. Eventually, hypercapnia and respiratory acidosis develop, leading to pulmonary artery vasoconstriction and cor pulmonale. With the ensuing hypoxemia, polycythemia, and increased CO2 retention, these patients have signs of right heart failure and are known as "blue bloaters."
Emphysema is defined by destruction of airways distal to the terminal bronchiole. Physiology of emphysema involves gradual destruction of alveolar septae and of the pulmonary capillary bed, leading to decreased ability to oxygenate blood. The body compensates with lowered cardiac output and hyperventilation. This V/Q mismatch results in relatively limited blood flow through a fairly well oxygenated lung with normal blood gases and pressures in the lung, in contrast to the situation in blue bloaters. Because of low cardiac output, however, the rest of the body suffers from tissue hypoxia and pulmonary cachexia. Eventually, these patients develop muscle wasting and weight loss and are identified as "pink puffers."
