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The trachea is approximately 10 to 12 cm in length and 2.5 cm in diameter. It extends from the lower border of the larynx to the carina, which marks the point where it bifurcates (splits) into the right and left primary bronchi which extend towards their respective lungs. The trachea is held open by a series of stacked C-shaped rings of cartilage which serve to reinforce and prevent the airway collapsing during inspiration and expiration. The oesophagus runs parallel to the trachea, slotting longitudinally into the long posterior groove created by the stacked C-shaped cartilaginous rings.

As in the nasal cavity and nasopharynx, the trachea is lined by a ciliated pseudostratified epithelium which sweeps contaminated mucus and particulates away from the lungs towards the pharynx to be swallowed. This mechanism is known as the mucociliary escalator because it functions in a similar manner to a mechanical stairway, continually clearing and cleaning the airway, reducing the risk of irritation and infection.

Figure 4.4 Branching structure of the respiratory tract