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
- PaO2 = 40 mm Hg 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
- O2 binds hemoglobin inside RBCs, maintaining a low PaO2
- 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)
- PAO2 = PaO2
- initially, PaO2 rises only slightly along length of pulmonary capillary
- partial pressure of O2 (PAO2) in alveolar air is constant along length of capillary
- 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
- prevents O2 equilibration and maintains O2 partial pressure gradient along entire length of capillary → diffusion-limited process
- interstitium thickens → ↑ O2 diffusion distance → ↓ O2 diffusion rate
- e.g., pulmonary fibrosis