Light micrograph showing a cross-section of a bronchiole and surrounding alveoli (air sacs) in the human lung. At 0.5-1.0 millimetre in diameter, bronchioles are the smallest divisions of the brochial tree, the trunk of which is the trachea, the windpipe. Bronchioles terminate in clusters of alveoli, where the exchange of gases with the blood circulation occurs. The heavily-folded structure of the bronchiolar epithelium is clearly visible. The diameter of the tiny airway is controlled by a ring of smooth muscle which encircles the columnar epithelium. Magnification: x40 at 35mm size. Reference: UMICROCOSMOS u, figure 2.35, page 33.