airway resistance
Airway resistance is how much the airways oppose the flow of air, like the friction water feels running through a pipe. Wide, open airways let air slip through easily (low resistance); narrow or clogged airways make it a struggle (high resistance).
Resistance depends dramatically on airway width. By the physics of flow through a tube, resistance climbs steeply as the radius shrinks — roughly with the fourth power of the radius — so even modest narrowing from muscle spasm, swelling, or mucus can multiply resistance many times over.
Counterintuitively, most resistance in healthy lungs lies in the medium-sized bronchi, not the tiniest bronchioles. Although each small bronchiole is narrow, there are so many of them arranged in parallel that their combined cross-section is huge, so they contribute little resistance — until disease widely narrows them.
High airway resistance is the defining mechanical fault of obstructive diseases like asthma and COPD. It makes breathing out especially laborious, produces wheeze as air whistles through narrowed passages, and raises the work of breathing.
During an asthma attack, smooth-muscle spasm narrows the bronchi and resistance soars; a bronchodilator relaxes that muscle, widening the airways and dropping resistance back down.
Bronchospasm raises resistance; bronchodilators lower it.
Because the small airways normally contribute so little resistance, early disease there can be widespread before total resistance budges much — which is why the small airways are sometimes called the lung's silent zone.