This 10-cm tube extends from the larynx to the primary bronchi. It is lined by respiratory epithelium, and its lamina propria contains mixed seromucous glands that open onto its lumen. Its most characteristic feature is the presence of 16-20 C-shaped cartilage rings whose open ends are directed posteriorly. The opening is bridged by a fibroelastic ligament that prevents overdistension as well as by smooth muscle bundles (tracbealis muscle) that constrict the lumen and increase the force of air flow during coughing and forced expiration.
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