primary bronchus, one of the two main air passages that branch from the trachea and convey air to the lungs as part of the respiratory system. The right primary bronchus enters the right lung nearly opposite the fifth thoracic vertebra. The left primary bronchus divides into bronchi for the superior and anterior lobes of the lung. The bronchi, like the trachea, are composed of rings of hyaline cartilage, fibrous tissue, mucous membrane, and glands. The right primary bronchus has a more direct extension of the trachea than the left. Hence foreign objects entering the trachea usually drop into the right bronchus rather than the left. See also bronchial tree.
