M04.07.008 Airway Resistance and Its Determinants

Radius of an Airway

The majority of airway resistance in the human respiratory system is concentrated in the first and second bronchi.

• Bronchoconstriction:
  • Stimulated by parasympathetic nerves.
  • Mediated by M3 receptors.
  • Leads to increased airway secretions.
• Bronchodilation:
  • Induced by circulating catecholamines, primarily epinephrine.
  • Mediated by β2 receptors.

Mechanical Effect of Lung Volume

As lung volume increases, airway resistance decreases. This phenomenon is illustrated in Figure V-114-1.

• Pneumothorax: Higher negative intrathoracic pressure (IPP) at higher lung volumes expands small airways, reducing resistance.
• Radial traction: Alveolar expansion pulls open small airways, decreasing resistance.

Points to Remember

• The first and second bronchi contribute most to airway resistance.
• Parasympathetic stimulation causes bronchoconstriction, while catecholamines induce bronchodilation.
• Lung volume influences airway resistance: higher volumes lead to lower resistance due to pneumothorax and radial traction.

Bibliography

• West, J. B. (2008). Respiratory Physiology: The Essentials. 8th ed. Lippincott Williams & Wilkins.
• Guyton, A. C., & Hall, J. E. (2011). Textbook of Medical Physiology. 12th ed. Elsevier.

Note: To access the specific figure V-114-1, you may need to consult the original sources (West or Guyton & Hall) or contact the relevant publisher.