Lung bronchiole. Light micrograph of a section through lung tissue and a bronchiole. Bronchi form branches from the trachea and gradually diminish in diameter with increasing branching into bronchioles within the lung. Bronchioles are recognized by the decreasing and eventual loss of the 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: x50 when printed at 10 centimetres across.