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