The cilia lining the upper respiratory passages (superior to the larynx) beat inferiorly, whereas as the cilia lining the lower respiratory passages (larynx and inferior) beat superiorly. what is the functional reason for this difference?

The cilia in the upper respiratory tract move mucus  down toward the pharynx whereas the cilia in the  lower respiratory tract move them up toward the pharynx.

Explanation:

Through the internal nares, the air enters nasopharynx from the nasal cavity. This air has dust-laden mucus. The nasopharynx is lined with ciliated pseudostratified columnar epithelium. These cilia move the mucus down toward the most inferior part of the pharynx. The lining of the larynx inferior to the vocal folds is made up of ciliated pseudostratified columnar epithelium and have ciliated columnar cells, goblet cells, and basal cells.

The goblet cells serve to produce and secrete mucus. The mucus from goblet cells helps trap dust that was not removed in the upper passages. Therefore, the cilia in the upper respiratory tract move mucus and trapped particles down toward the pharynx whereas the cilia in the lower respiratory tract move them up toward the pharynx.

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