Cilia: The Respiratory Tract's Microscopic Cleaning System

Structure & Function of Respiratory Cilia

Cilia are microscopic, hair-like projections (5–10 µm long, 0.2 µm diameter) on the apical surface of pseudostratified ciliated columnar epithelium lining the trachea, bronchi, and bronchioles (down to terminal bronchioles).

Ultrastructure (9+2 Arrangement)

• 9 outer microtubule doublets + 2 central singlets.
• Dynein arms (ATPase) generate sliding → bending stroke.
• Coordinated metachronal wave → directional mucus transport.

Mucociliary Clearance (MCC) – The "Escalator"

1. Mucus Layer: Gel layer (mucins MUC5AC, MUC5B) traps particles, pathogens, allergens.
2. Periciliary Layer (PCL): Low-viscosity fluid ~7 µm deep – cilia beat within this.
3. Effective Stroke: Cilia extend, push mucus cephalad (toward pharynx).
4. Recovery Stroke: Cilia bend flat in PCL, return without moving mucus.
5. Clearance Rate: ~5–20 mm/min trachea; ~1–2 mm/min small airways.
6. Terminal: Mucus swallowed or expectorated.

Factors Impairing Ciliary Function

• Smoking/Vaping: Ciliary paralysis, loss, squamous metaplasia.
• Infections: Viruses (influenza, COVID), bacteria (pertussis) – ciliostasis, stripping.
• Pollutants: SO₂, NO₂, ozone, particulate matter.
• Genetic: Primary Ciliary Dyskinesia (PCD) – dynein arm defects → chronic sinusitis, bronchiectasis, situs inversus (Kartagener), infertility.
• Dehydration: Thickened mucus, PCL depletion → cilia cannot beat effectively.
• Anesthesia/Intubation: Transient dysfunction.

Clinical Relevance

• Impaired MCC → recurrent infections, chronic cough, bronchiectasis.
• Nasal nitric oxide (nNO) low in PCD – screening test.
• High-speed video microscopy / electron microscopy for diagnosis.

Cilia are the lung's first line of defense. Protect them: don't smoke, hydrate, avoid pollutants.