The document discusses the histology and structure of the respiratory system. It describes the trachea and bronchi, which make up the air passages in the lungs. The trachea leads from the back of the mouth to the bronchi. Both structures have walls consisting of cartilage, glandular tissue, connective tissue, elastic fibers, smooth muscle, and blood vessels. The inner lining contains ciliated epithelial cells and goblet cells that secrete mucus. The C-shaped cartilage rings in the trachea help push objects out if something blocks the airway.