Lung bronchus. Light micrograph of a section through lung tissue and a 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 is seen between the cartilage and the epithelial lining of the bronchus. Contraction and relaxation of the smooth muscle modifies the shape and diameter of the bronchial epithelial tissue. Magnification: x40 when printed at 10 centimetres across.
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