Airflow, Pressure, and Resistance

Overview

• Airflow, Pressure, and Resistance
  • Q = ΔP / R
    • airflow is directly proportional to pressure difference between mouth/nose (atmospheric) and alveoli
    • airflow is inversely proportional to airway resistance
• Airway Resistance 
  • R = 8ηl / πr⁴
    • η = viscosity of inspired air
    • l = length of airway
    • r = radius of airway
  • total resistance depends on the aggregate cross-sectional area of a zone 
    • highest in the bronchi between the 4th and 8th generations of branching
• Changes in Airway Resistance
  • changes in airway diameter provide major mechanism for altering airway resistance and airflow
  • Autonomic Nervous System
    • sympathetic stimulation
      • bronchial smooth muscle is innervated by sympathetic adrenergic nerve fibers
      • sympathetic stimulation → (+) β2 receptors → relaxation → ↑ airway diameter, ↓ resistance to airflow
      • β2 agonists (e.g., epinephrine, isoproterenol, albuterol) relax bronchial smooth muscle and dilate airway in treatment of asthma 
    • parasympathetic stimulation
      • bronchial smooth muscle is innervated by parasympathetic cholinergic nerve fibers
      • parasympathetic stimulation → (+) muscarinic receptors → constriction → ↓ airway diameter, ↑ resistance to airflow
• Pulmonary Vascular Resistance (PVR)
  • PVR = [P(pulmonary artery) – P(left atrium)] / cardiac output
    • P(pulmonary artery) is pressure in pulmonary arter
    • (left atrium) is pulmonary capillary wedge pressure