Updated: 4/13/2017

Diffusion-Limited and Perfusion-Limited Gas Exchange

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Overview
  • Describes gas exchange across  alveolar-capillary barrier
    • illustrated by partial pressure / time graph
  • There are two main types
    • Diffusion-Limited Gas Exchange
      • applies to CO
    • Perfusion-Limited Gas Exchange
      • applies to N2O, CO2
Diffusion-Limited Gas Exchange (CO)
  • Gas exchange across alveolar-capillary barrier is limited by diffusion process
  • Net diffusion into pulmonary capillary depends on magnitude of partial pressure gradient
  • Example: CO
    • partial pressure of CO (PACO) in alveolar air is constant along length of capillary
    • partial pressure of CO (PaCO) in capillary blood is zero at beginning of pulmonary capillary
      • largest partial pressure gradient of CO and largest driving force for diffusion of CO from alveolar air into capillary blood at beginning of pulmonary capillary
    • CO diffuses into capillary blood moving along length of pulmonary capillary
      • PaCO rises only slightly along length of pulmonary capillary
        • CO avidly binds hemoglobin inside RBCs, maintaining a low PaCO
          • only a free, dissolved gas in capillary blood causes a partial pressure
    • CO does not equilibrate by end of capillary regardless of the amount of blood flow
      • partial pressure gradient of CO is maintained along entire length of capillary
        • maintains driving force for net diffusion of CO
Perfusion-Limited Gas Exchange (N2O, CO2)
  • Gas exchange across alveolar-capillary barrier is limited by blood flow through pulmonary capillaries (perfusion)
  • e.g., N2O
    • partial pressure of N2O (PAN2O) in alveolar air is constant along length of capillary
    • partial pressure of N2O (PaN2O) in capillary blood is zero at beginning of pulmonary capillary
      • largest partial pressure gradient of N2O and largest driving force for diffusion of N2O from alveolar air into capillary blood at beginning of pulmonary capillary
    • N2O diffuses into capillary blood moving along length of pulmonary capillary
      • PaN2O rises rapidly along length of pulmonary capillary
        • N2O remains a free, dissolved gas in capillary blood and causes a partial pressure
    • N2O equilibrates early along length of capillary
      • partial pressure gradient of N2O is not maintained along length of capillary
        • eliminates driving force for net diffusion of N2O
    • only means for increasing net diffusion of N2O is by increasing blood flow through pulmonary capillaries (perfusion)

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