Airway resistance !
- Concept
- Exam priority: medium
The narrower a tube, the harder it is to push air through. Halving the radius multiplies resistance by 16.
Updated 2026-11-01
Also called: Poiseuille's law in the airways, Bronchoconstriction
See it in 3D: Trachea · Main bronchi · Lobar bronchi · Segmental bronchi
What it is
Airway resistance is the opposition to airflow; by Poiseuille's law it depends on the radius to the fourth power. Most resistance sits in the medium-sized bronchi, not the bronchioles, because bronchioles are so numerous and in parallel. Smooth muscle tone, mucus, edema and lung volume change it.
How it works, step by step
- In the trachea and large bronchi flow is fast and turbulent.
- In the medium bronchi total cross-section is still small, so resistance concentrates there.
- Further down, branches multiply, total area grows and the resistance of each level falls.
- During a forced blow, positive pleural pressure squeezes the cartilage-free small airways and limits flow.
Equations
- ΔP
- mouth-to-alveolus pressure difference
- R
- airway resistance
- η
- gas viscosity
- l
- tube length
- r
- radius
Holds for laminar flow; in the trachea flow is turbulent and resistance is even higher.
Regulation
- −Sympathetic / epinephrine (β2): Relaxes bronchial smooth muscle: resistance falls.
- +Vagus / acetylcholine (M3): Contracts smooth muscle and increases mucus.
- +Histamine, leukotrienes, irritants: Bronchoconstriction and mucosal edema (asthma, allergy).
- −High lung volume: Radial traction from the parenchyma opens the airways.
Clinical notes
- Asthma: bronchospasm, edema and mucus shrink the radius; wheeze and prolonged expiration.
- Wheeze (intrathoracic airways) and stridor (extrathoracic airway) come from turbulent flow through a narrowed airway.
Test yourself
Think of the answer, then tap the question to see it.
If asthma narrows a bronchus's radius by 50%, how much does its resistance rise?
About 16-fold (2⁴), because resistance depends on the radius to the fourth power.
Why do the bronchioles, though so thin, add little total resistance?
There are thousands in parallel: their total cross-sectional area is huge and their combined resistance is low.
Why does an asthmatic breathe at high lung volumes?
At higher volume the parenchyma pulls the airways open, lowering resistance, even though it costs more elastic work.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Obstructive and restrictive patterns, and flow–volume loops
- Lung and chest wall compliance
- The work of breathing
- Mechanics of breathing: pressures and muscles
- Airway defenses, cough and mucociliary clearance
Related conditions
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: 22.3 The Process of Breathing (OpenStax (Rice University), 2022); Asthma (StatPearls (NCBI Bookshelf), 2024); Anatomy, Thorax, Tracheobronchial Tree (StatPearls (NCBI Bookshelf), 2024); 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.