Airway defenses, cough and mucociliary clearance !

Concept
For completeness

The nose filters, mucus traps, cilia sweep upward and cough throws it out: that is how the lung stays clean.

Updated 2026-11-01

Also called: Mucociliary escalator, Cough reflex

See it in 3D: Nasal cavity · Trachea · Main bronchi · Segmental bronchi · Epiglottis · Vocal folds

What it is

The airways breathe thousands of liters of dusty, microbe-laden air each day. The nose warms, humidifies and filters; particles that get past are trapped in mucus and swept by cilia toward the pharynx; in the alveoli, macrophages take over. Cough and sneeze are reflexes that forcefully expel irritants. When clearance fails (smoking, cystic fibrosis, ciliary dyskinesia) secretions pile up and infections recur.

How it works, step by step

The turbinates make airflow turbulent: large particles hit the moist nasal mucosa and stick.
In trachea and bronchi, medium particles settle on the mucus by impaction and sedimentation.
Cilia beat in coordination in the periciliary layer and move the mucus sheet up to the larynx, where it is swallowed or expelled.
If something irritates the larynx or trachea, vagal receptors trigger cough: inspiration, glottic closure and expiratory contraction.
Intrathoracic pressure rises sharply; when the glottis opens, air leaves at high speed and drags out mucus and foreign material.
In the alveoli, macrophages engulf fine particles and microbes and move to the escalator or the lymphatics.

When the defenses fail

Failure	Mechanism	Consequence
Smoking	Ciliary paralysis and loss, mucous gland hyperplasia	Chronic productive cough, COPD
Cystic fibrosis	Periciliary layer dehydrated by faulty CFTR	Thick mucus, infections, bronchiectasis
Primary ciliary dyskinesia	Cilia that do not beat	Sinusitis, bronchiectasis, sometimes situs inversus
Weak cough or incompetent glottis	Neuromuscular disease, sedation, stroke	Aspiration and pneumonia

Clinical notes

Urban pollution and wood smoke in rural Colombia impair mucociliary clearance and cause COPD in non-smokers, especially women.
Stroke patient with dysphagia: weak cough and silent aspiration explain recurrent pneumonia.
Nursing: repositioning, hydration and chest physiotherapy help mobilize secretions.

Test yourself

Think of the answer, then tap the question to see it.

Why does bronchiectasis develop in cystic fibrosis if the cilia are normal?

Faulty CFTR dehydrates the periciliary layer; cilia are trapped in thick mucus they cannot move, and chronic infection destroys the bronchial wall.

Which step of the cough reflex fails in an intubated patient?

Glottic closure: without it the high pressure before sudden opening cannot build, and cough is ineffective.

Why do very fine particles, such as smoke, reach the alveoli?

They are too small to impact on the mucosa or settle in the conducting airways; they ride the airflow to the alveoli, where macrophages must deal with them.

Why can an ACE inhibitor cause a persistent dry cough?

Angiotensin-converting enzyme also breaks down bradykinin; blocking it lets bradykinin build up and sensitize airway cough receptors.

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See it in 3D

The structures of Durgical's 3D anatomy where it happens.

In Pharmacology

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Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 22.1 Organs and Structures of the Respiratory System (OpenStax (Rice University), 2022); Anatomy, Thorax, Tracheobronchial Tree (StatPearls (NCBI Bookshelf), 2026); Bronchiectasis (StatPearls (NCBI Bookshelf), 2026); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 38). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.