Updated: 8/23/2017

Diffusion-Limited and Perfusion-Limited Oxygen Transport

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                                       Diffusion-Limited and Perfusion-Limited Oxygen Transport
  • Perfusion-limited O2 transport (normal conditions)
    • under normal conditions, O2 diffusion from alveolar air into pulmonary capillary blood is perfusion-limited
    • net O2 diffusion into pulmonary capillary depends on magnitude of partial pressure gradient
      • partial pressure of O2 (PAO2) in alveolar air is constant along length of capillary
        • PAO2 = 100 mm Hg
      • partial pressure of O2 (PaO2) in capillary blood
        • PaO2 = 40 mm Hg at beginning of pulmonary capillary
          • PaO2 reflects composition of mixed venous blood
        • large partial pressure gradient of O2 drives O2 diffusion from alveolar air into capillary blood at beginning of pulmonary capillary
      • O2 diffuses from alveolar air into capillary blood moving along length of pulmonary capillary
        • initially, PaO2 rises only slightly along length of pulmonary capillary
          • O2 binds hemoglobin inside RBCs, maintaining a low PaO2
            • only a free, dissolved gas in capillary blood causes a partial pressure
        • eventually, PaO2 rises significantly along length of pulmonary capillary
          • Hemoglobin eventually is saturated 
          • O2 equilibration occurs approximately one-third of the distance along length of capillary
            • PAO2 = PaO2
              • eliminates driving force for net diffusion of O2
              • only means for increasing net diffusion of O2 is by increasing blood flow through pulmonary capillaries (perfusion)
  • Diffusion-limited O2 transport (pathologic conditions; exercise)
    • under conditions of strenuous exercise, O2 diffusion from alveolar air into pulmonary capillary blood is diffusion-limited 
    • under certain pathologic conditions, O2 diffusion from alveolar air into pulmonary capillary blood is diffusion-limited
      • e.g., emphysema, pulmonary fibrosis
    • net O2 diffusion into pulmonary capillary depends on magnitude of partial pressure gradient
      • e.g., pulmonary fibrosis
        • interstitium thickens ↑ O2 diffusion distance ↓ O2 diffusion rate
          • prevents O2 equilibration and maintains O2 partial pressure gradient along entire length of capillary diffusion-limited process
            • however, ↓ net O2 diffusion ↓ PaO2 in systemic arterial blood
 

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