Mucociliary clearance in the airways
Mucociliary clearance is the innate (built-in) physical defence in which mucus traps inhaled microbes and cilia, tiny hair-like projections, sweep it up towards the throat.
Remember
Mucus traps microbes and coordinated ciliary beating carries them towards the throat, so when cilia are damaged, as by cigarette smoke, trapped microbes are cleared less well and stay in the airways for longer.
Memory aid
mucus is the flypaper and the cilia are the conveyor belt that carries the paper away
Picture the mucus as flypaper that catches microbes and the cilia as a conveyor belt that carries it away towards the throat.
The lining of the airways (the respiratory epithelium, a sheet of lining cells) contains goblet cells and ciliated cells. Goblet cells make mucus, the sticky fluid that traps anything that lands on it, including inhaled dust and microbes. The ciliated cells carry cilia, tiny hair-like projections that beat together in one direction.
This system is called the mucociliary escalator. Mucus catches the microbes, and the beating cilia carry the mucus upwards towards the pharynx (the back of the throat). There it is coughed out or swallowed, and swallowed microbes can be destroyed in the stomach.
This is an innate defence, meaning it is built in and fast, with no memory of earlier infections, so it works on any microbe straight away. It is physical because it removes microbes by moving them rather than by killing them. It acts in the conducting airways (the tubes that carry air towards the lungs), so it works before microbes reach the alveoli, the tiny air sacs where gas exchange happens. Alveolar macrophages, cells that engulf and digest microbes in the air sacs, act once microbes have arrived.
One weak point is the cilia. Cigarette smoke shortens cilia and slows their beating, while goblet cells often increase in number and make more mucus, so there is more mucus to move and less ability to move it. Respiratory viruses target ciliated cells, so after a viral infection mucus and microbes can sit in the airway for longer. In primary ciliary dyskinesia, an inherited condition in which the moving (motile) cilia are faulty, mucus stagnates and infections keep returning.
Key facts
- Trap
- mucus, made by goblet cells, traps microbes and particles that land on the airway lining.
- Carry
- cilia beat in step to sweep the mucus towards the throat, where it is swallowed or coughed out.
- Innate and physical
- it moves microbes out rather than killing them, and it needs no earlier exposure.
- Smoking
- cigarette smoke gives shorter cilia that beat more slowly, so mucociliary clearance is impaired.
- Viruses
- respiratory viruses target ciliated cells and slow clearance, so mucus and microbes can linger.
- Primary ciliary dyskinesia
- inherited faulty motile cilia; lifelong airway infections with normal antibodies and normal neutrophil function suggest failed mucociliary clearance, as in this condition.
Exam traps
- Mucus production by goblet cells is not what smoking impairs, because smokers often have more goblet cells and make more mucus; the failure lies in the cilia that should move it.
- Alveolar macrophages do not prevent pathogens reaching the alveoli, because they sit in the alveoli and deal with organisms that have already arrived there.
- Low secretory IgA, the antibody in mucus, does not explain delayed nasal mucus clearance, because it would not slow the movement of mucus, and normal immunoglobulin levels argue against an antibody defect.
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