Our airway walls are made up of cartilage, smooth muscle (one of the three muscle types), mucus-producing glands, connective tissue, and epithelium (the type of tissue that covers all outer and inner body surfaces, including the lining of vessels and other small cavities). Because the relative amounts of these components changein healthy lungsas you move into the lungs along the airway branches, the large, medium, and small airways are distinctive in composition as well as in size.

Mucus

Connective tissue (the fibroelastic framework) kind of holds the overall structure together. Mucusthe layer of sticky fluid covering the lining of epithelial cellsis made up of water, large molecules of sugar and protein, and salts. It is technically called sputum. Mucus is produced by goblet cells in the lining of medium and large airways, and by mucous glands deep in the airway wall. (In a healthy nonsmoker, the proportion of goblet cells greatly decreases as the airways become smaller.) The mucus-producing goblet cells are interspersed among other epithelial cells with moving, hair-like projections called cilia. These two types of cells form a protective alliance that keeps the airways sterile. Mucus traps foreign particlesincluding bacteriathat enter the airways, then the cilia push both mucus and