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 login to view 1 more bullet partial pressure of O2 (PaO2) in capillary blood login to view 3 more bullets O2 diffuses from alveolar air into capillary blood moving along length of pulmonary capillary login to view 9 more bullets 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 login to view 3 more bullets