They have relatively large lumens that are lined by respiratory epithelium. There is a smooth muscle layer below the epithelium arranged as two ribbons of muscle that spiral in opposite directions. This smooth muscle layer contains seromucous glands. Irregularly arranged plates of hyaline cartilage surround the smooth muscle. These plates give structural support to the bronchus and maintain the patency of the lumen.
A bronchus has cartilage plates, smooth muscle, and mucus-secreting gland-cells in its wall. It also has lining cells with cilia departing towards the mouth; this removes dust and other small debris.