Airway resistance determinants

The Resistance Equation - Poiseuille's Playground

Airway resistance (R) is defined by Poiseuille's Law, highlighting the profound impact of airway radius.

• Poiseuille's Law: $R = \frac{8ηL}{πr^4}$

  • $η$ (eta): Viscosity of inspired gas.
  • $L$: Length of the airway.
  • $r$: Radius of the airway.

• Key Determinant: Airway radius ($r$) is the most critical factor. Resistance is inversely proportional to the radius to the fourth power ($R \propto 1/r^4$).

  • Halving the radius ↑ resistance by 16x.

⭐ Exam Favorite: The highest airway resistance is found in the medium-sized bronchi, not the terminal bronchioles. While individual bronchioles are narrow, their massive total cross-sectional area results in low overall resistance.

Airway Anatomy - Resistance Hotspots

• Resistance is governed by Poiseuille's Law: $R \propto \frac{1}{r^4}$.
  • Halving the radius (r) increases resistance by 16x.
• Primary resistance site: Medium-sized bronchi (generations 2-5).
  • Large airways have low resistance (wide radius).
  • Smallest airways (<2 mm) have low total resistance due to their massive cumulative cross-sectional area from parallel arrangement.

⭐ The majority of airway resistance (>50%) is not in the expected small bronchioles, but in the medium-sized bronchi. This is a frequent exam point testing the concept of parallel resistance.

Physiologic Control - Nerves & Stretch

• Autonomic Nervous System:

  • Parasympathetic (Vagal tone): Dominant neural control at rest.
    • ACh → M3 receptors → Bronchoconstriction & mucus secretion → ↑ Resistance.
    • Blocked by anticholinergics (e.g., Ipratropium).
  • Sympathetic:
    • Minimal direct innervation.
    • Circulating Epinephrine → β2 receptors → Bronchodilation → ↓ Resistance.
    • Target of β2-agonists (e.g., Albuterol).

• Lung Volume & Stretch:

  • ↑ Lung Volume → ↑ Airway caliber (radial traction) → ↓ Resistance.
  • ↓ Lung Volume → ↓ Airway caliber → ↑ Resistance.

⭐ At rest, a baseline level of smooth muscle tone is maintained by tonic vagal (parasympathetic) activity, making it the primary determinant of airway resistance.

Clinical Correlations - Wheezes & Cures

• Obstructive Diseases (Asthma, COPD): Defined by chronically ↑ airway resistance, leading to expiratory wheezes.
  • Caused by bronchoconstriction, airway inflammation, and mucus plugging.
• Pharmacologic Interventions: Target receptors controlling smooth muscle tone.
  • β2-Agonists (Albuterol): Stimulate Gs → ↑cAMP → bronchodilation (↓ Resistance).
  • Muscarinic Antagonists (Ipratropium): Block M3 (Gq) → ↓IP3/Ca²⁺ → prevent bronchoconstriction.
  • Corticosteroids (Fluticasone): Reduce inflammation, addressing the chronic component.

⭐ High-Yield: Leukotriene inhibitors (e.g., Montelukast) are crucial in aspirin-exacerbated respiratory disease (AERD) by blocking the pro-inflammatory leukotriene pathway.

High-Yield Points - ⚡ Biggest Takeaways

• Poiseuille's law is key: resistance is inversely proportional to the radius to the fourth power (r⁴), making the airway radius the primary determinant.
• The highest resistance is in the medium-sized bronchi, not the terminal bronchioles, which have a larger total cross-sectional area in parallel.
• Increased lung volumes decrease resistance by exerting radial traction on airways.
• Parasympathetic stimulation (ACh) causes bronchoconstriction, increasing resistance.
• Sympathetic stimulation (β2 agonists) causes bronchodilation, decreasing resistance.