USMLE

Breathing Mechanics

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Respiratory
  1. Respiratory Overview
  2. Airway Anatomy
  3. Alveoli
  4. Breathing Mechanics
  5. Gas Exchange in Tissues
  6. Nervous Control of Breathing
  7. Respiratory Defense Systems
  • Breathing/Respiration
    • Governed by changing differential pressures in chest cavity and airway
    • Inhalation 
      • 1. Diaphragm and intercostal muscles contract
        • Diaphragm moves downward, rib cage elevates to expand thoracic cavity
      • 2. Vacuum (negative pressure) in pleural cavity pulls lung 
        • Lungs are surrounded by thin, fluid-lined space called pleura, which contains the intrapleural space (pleural cavity)
        • Negative pressure (vacuum) of intrapleural space pulls lung in contact with expanding thoracic wall 
      • 3. Lungs expand to fill space
      • 4. Vacuum (negative pressure) pulls air into lungs
        • Alveoli expand due to decrease in negative pressure and influx of air 
        • Gas exchange between capillaries and alveoli of lung
    • Exhalation 
      • This is a passive process, dictated by natural elasticity of tissue
        • Forced expiration (ex: during exercise) involves abdominal muscles and distinct set of intercostal muscles to shrink chest cavity 
      • 1. Diaphragm relaxes and intercostal muscles relax
        • Diaphragm moves up and rib cage moves down
      • 2. Elastic recoil of alveoli pulls pleural cavity and thoracic cavity inwards
      • 3. Lung volume shrinks
      • 4. Increased pressure in lung pushes air out
  • Important Terms
    • Tidal volume = volume of air that moves into lungs during each inspiration
      • Some air doesn’t reach alveoli = stuck in respiratory dead space
      • Respiratory rate = number of breaths per minute