Airway Resistance

Expansion

Although each small airway is narrow, they are so numerous that their combined cross-sectional area is enormous. Resistance is therefore dominated by the medium-sized bronchi, roughly the first eight generations, with airways under 2 mm contributing only about 10 to 20 per cent of the total.

Increased by

• Bronchoconstriction: parasympathetic (vagal, M3) stimulation, histamine, leukotrienes, cold air
• Reduced lung volume, since airways are tethered open by surrounding parenchyma
• Secretions, oedema and mucosal inflammation
• Increased gas density, as at depth

Decreased by

• Sympathetic beta-2 stimulation and circulating adrenaline
• Increased lung volume
• Low density gas such as heliox

The “silent zone” concept follows directly: small airways disease can be extensive before spirometry detects it, which is why measures such as mid-expiratory flow are more sensitive early.