cilia

Cilia are microscopic, hairlike projections that stick up from the surface of airway-lining cells, like a dense field of grass bending in a steady wind. Except here the wind is internal: each tiny hair beats back and forth on its own, and together they push the overlying mucus in one direction — upward and out.

Each ciliated cell carries roughly 200 cilia, and a single cilium contains an exquisite internal scaffold called the axoneme, built from nine pairs of microtubules around a central pair. Tiny motor proteins (dyneins) slide these microtubules against one another, producing the rhythmic bend. The beat is fast — around ten to twenty cycles per second — and neighboring cilia coordinate so the motion travels as a wave rather than random flailing.

Cilia are the engine of mucociliary clearance, so their health is central to airway defense. Heat, drying, smoke, viral infection, and certain pollutants slow or stop the beat, while genetic faults in the axoneme (as in primary ciliary dyskinesia) leave cilia immotile or disorganized from birth, causing lifelong clearance problems and recurrent infection.