Two layers that work as one
Run your eye along the inside of a bronchus and you would find it carpeted with respiratory mucosa. On top sits a thin film of mucus in two layers: a watery, slippery lower layer (the periciliary or sol layer) and a sticky, gel-like upper blanket. Particles and microbes land on the sticky gel and get glued in place — like flies on flypaper.
Beneath the gel, embedded in the watery layer, are the cilia: each lining cell carries around 200 of these tiny hair-like projections. They beat in a coordinated wave, roughly 10–20 times per second, with a fast power stroke that brushes the tips against the gel and pushes it along, then a slow recovery stroke below the gel. The result is that the whole sticky blanket — with its trapped cargo — creeps steadily toward the throat.
The escalator goes up
Because the cilia all beat toward the mouth, the whole system behaves like an escalator — the mucociliary escalator. In the larger airways the mucus moves at roughly a few millimeters per minute. Mucus from the right and left lungs travels up the bronchi, reaches the trachea, and arrives at the throat, where it is swallowed without you noticing (and harmlessly destroyed by stomach acid) or cleared by a small throat-clear. Most of the dirt in the air you breathe ends up quietly in your stomach.
Where does the mucus come from? Two main sources: scattered goblet cells in the airway epithelium that secrete the gel, and deeper mucous glands in the airway wall. A healthy adult makes a surprisingly small amount — perhaps 10–100 milliliters a day — and clears almost all of it silently. You only see it as sputum when production surges (infection, smoke) or clearance slows.
When the escalator stalls
- Cigarette smoke paralyzes and then destroys cilia and stimulates extra mucus. The escalator slows just as the load rises — which is why smokers cough, especially in the morning after a night of stalled clearance.
- Dehydration of the watery layer (in cystic fibrosis, or when the airway surface dries out) thickens the gel so cilia can no longer move it; mucus stagnates and breeds infection.
- Broken cilia — as in primary ciliary dyskinesia, a genetic disorder — leave the cilia unable to beat properly from birth, causing lifelong infections (covered in the next-but-one guide).