Chronic bronchitis is the name for chronic inflammation of the mucous membranes of the bronchial tree. Label 1 shows infiltration of inflammatory immune cells into lung tissue. Prolonged exposure to atmospheric irritants, commonly cigarette smoke, causes inflammation of airways walls and hypertrophy (overgrowth) of mucous glands. Excess mucus from hypertrophied mucous glands (Label 2) eventually blocks the airways. Most patients with chronic bronchitis also have variable amounts of alveolar destruction. Label 3 highlights a blood vessel filled with red blood cells.