The airways are lined with ciliated pseudostratified columnar epithelia. Goblet cells of the trachea and bronchi secrete mucus that traps particles. A watery saline solution covers the cilia, so they are able to make sweeping motions upward. This structure is the mucociliary escalator by which particles trapped in the mucus are carried up and out by the epithelial cilia.
The diameter of the airways can change under some circumstances. Bronchoconstriction is the result of smooth muscle contraction. It deceases bronchiole diameter. Irritant receptors in the airway mucosa respond to particles and noxious gases with bronchoconstriction. Bronchodilation is increased bronchiole diameter.
Making a Difference in the World 