Diffusion of oxygen from gas pockets to capillaries

Exit rate of O₂ from subcutaneous gas pockets in unanesthetized rats was studied at 1 atm pressure, breathing air or pure O₂, and in a hyperbaric environment, breathing 1 atm of O₂, remainder N₂. The findings were (a) rats had lower O₂ exit rate ( V ̇_O2) when breathing O₂ than when breathing air and (b) at low P_O2, curves of V ̇_O2 vs pocket P_O2 were concave downward but changed to linear or concave upward above 1 atm. These data are consistent with a model which assumes that O₂ diffuses through tissue until it meets a capillary and that the means by which blood carries O₂ changes as pocket P_O2 increases: Hb does not become saturated at low P_O2, but does at higher P_O2, and physical solution becomes important at very high P_O2. The success of the model implies that for diffusion of gases, the tissue-capillary milieu approximates a homogeneous medium in which both diffusion and clearance take place at the same time and at every location.