Updated: 8/10/2020

Airflow, Pressure, and Resistance

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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 artery
      • P(left atrium) is pulmonary capillary wedge pressure

       

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