Mucociliary clearance
Also known as: mucociliary escalator, airway surface liquid, ciliary beat
The "mucociliary escalator": a mucus blanket that traps inhaled particles and is propelled by coordinated ciliary beating towards the pharynx, where it is swallowed or expectorated.
Overview
The airway surface liquid has two layers: a watery periciliary (sol) layer about as deep as the cilia are long (≈7 µm), in which the cilia beat freely, and an overlying mucus (gel) layer of mucins (MUC5AC, MUC5B) secreted by goblet cells and submucosal glands. Each ciliated cell bears ≈200–300 cilia beating at ≈10–20 Hz in coordinated metachronal waves; the tips of the cilia catch the gel layer only during the forward (effective) stroke, carrying it cranially at several millimetres per minute in the trachea. Mucus from the nose is moved backwards to the pharynx.
Particle deposition depends on size: particles > ≈10 µm are filtered in the nose; ≈2–10 µm impact in the trachea and bronchi and are removed by the escalator; ≈0.5–2 µm can reach the alveoli, which lack cilia and are cleared by alveolar macrophages. Hydration of the periciliary layer is maintained by CFTR-mediated Cl− secretion and ENaC-mediated Na+ absorption.
- Location
Ciliated conducting airways from the nose to the bronchioles.
Parts & subdivisions
- Motile 9+2 axonemes powered by dynein.
- Ciliated and goblet cells.
Function
- Removes inhaled particles and microbes from the conducting airways.
- Carries antimicrobial factors (lysozyme, lactoferrin, defensins, secretory IgA).
Clinical significance
Smoking
Tobacco smoke slows and then destroys cilia and induces goblet-cell hyperplasia, so mucus accumulates ("smoker's cough"), predisposing to infection and chronic bronchitis.
Cystic fibrosis and primary ciliary dyskinesia
In CF dehydrated airway surface liquid collapses the periciliary layer so cilia cannot move mucus; in PCD the cilia themselves are immotile or dyskinetic. Both cause chronic infection and bronchiectasis.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.