Lung bronchus. Light micrograph of a section through a human bronchus. Bronchi form branches from the trachea and gradually diminish in diameter with increasing branching within the lung. Bronchi are recognized by crescent-shaped, incomplete rings of cartilage that ensure the patency of the airway. A circumferential ring of smooth muscle (brown) is seen underlying the epithelial lining ( red) of the bronchus. Contraction and relaxation of the smooth muscle modifies the shape and diameter of the bronchial epithelial tissue. Mucous secreting seromucous glands are also present. (cyan). Magnification: x150 when printed at 10 centimetres across.