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