Interaction of CO and O₂ with hemoglobin in perfused tissue adjacent to gas pockets

When oxygen and carbon monoxide diffuse simultaneously out of artificially-formed gas pockets in living rats, the two gases interact: exit rate of O₂ is lower in the presence of CO than in its absence, and exit rate of CO is unexpectedly low at high P_O2. The interaction is assumed to be due to competition for hemoglobin in the capillary blood which perfuses the tissue surrounding the gas pocket. The data are approximately consistent with a mathematic model based on the idea that the pocket gases diffuse out into the tissues and create a partial pressure gradient which extends for a considerable distance beyond the gas-tissue interface. Discrepancy between theory and observation can be explained by the idea that the displacement of O₂ from Hb by CO raises the O₂ tension inside the capillary high enough to inhibit further CO uptake by Hb, even though P_CO and P_O2 are about equal in the gas pocket. Thus there are tissue-to-capillary gradients as well as a pocket-to-tissue gradient.