Cilia, tiny muscular, hair-like projections on the cells that line the airway, are one of the respiratory system's defense mechanisms. Cilia propel a liquid layer of mucus that covers the airways.
The mucus layer traps pathogens (potentially infectious microorganisms) and other particles, preventing them from reaching the lungs.
Cilia beat more than 1,000 times a minute, moving the mucus that lines the trachea upwards about 0.5 to 1 centimeter per minute (0.197 to 0.4 inch per minute). Pathogens and particles that are trapped on the mucus layer are coughed out or moved to the mouth and swallowed.
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