Expansion
Cilia beat in a watery sol layer, moving an overlying gel layer of mucus upwards
Expansion
The airway lining has two layers:
- Sol (periciliary) layer - watery, in which the cilia beat freely
- Gel (mucus) layer - viscous, floating on top and trapping particles
Cilia beat about 12 to 15 times per second with a fast forward stroke that engages the gel layer and a slow recovery stroke within the sol layer, propelling mucus towards the pharynx at about 1 cm per minute.
Impaired by
- Smoking, which slows and destroys cilia
- Dehydration and dry inspired gas, which is why humidification matters in intubated patients
- Cystic fibrosis, where defective chloride transport dehydrates the sol layer
- Primary ciliary dyskinesia (Kartagener syndrome): bronchiectasis, sinusitis, situs inversus and infertility
- General anaesthesia, high inspired oxygen and endotracheal tubes
Particles above 10 micrometres are filtered in the nose; those of 2 to 10 micrometres deposit in airways and are cleared this way; those under 2 micrometres reach alveoli and are handled by macrophages instead.