bronchus

A bronchus is a large branch of the airway tree — think of the trachea as the trunk and the bronchi as the first major limbs dividing off toward each lung. The two main bronchi then split again and again into ever-smaller branches inside the lungs.

Like the trachea, bronchi are held open by cartilage (here in plates rather than neat rings) and are wrapped in smooth muscle whose tone can widen or narrow them. Their lining carries mucus and cilia that keep sweeping debris upward. They are still conducting airways: they move air but do not themselves exchange gas.

Bronchi are central to common lung disease. In asthma their muscle tightens (bronchospasm); in chronic bronchitis their lining is inflamed and over-produces mucus; and they are the highways a bronchoscope travels to inspect the airways or take samples.