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Updated: Aug 23 2017

Diffusion-Limited and Perfusion-Limited Oxygen Transport

  • Overview
    • 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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