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Lung airways in bronchitis, comparative illustrations. The artworks show cross-sections through a normal airway (top) and an inflamed, constricted airway (bottom). At left, the process of mucus production is shown in vertical section through the airway walls. The airways have a ring of smooth muscle and a thin layer of endothelial tissue. In the inflamed airway, the smooth muscle has contracted, and endothelial cell debris and mucus are partially blocking the airway. This is seen in conditions such as chronic bronchitis. Drugs to treat the condition reduce the inflammation and widen the airways, making it easier to breathe. (KEYSTONE/SCIENCE PHOTO LIBRARY/MAURIZIO DE ANGELIS/SCIENCE PHOTO LIBRARY)