CO2 binds to haemoglobin irrespective of O2 as the sites are remote from each other, however, I do seem to remember that this is one of the mechanisms by which the bhor shift occurs. Thus the bound CO2 affects the binding site of the O2 and reduces the bond strength.
Frank's post stirred up some thoughts in my head about the time it takes to get CO2 from the body to the lungs. If memory serves (which it may not
) then it takes 5-10 minutes for a red blood cell to make a return trip from the lungs to the legs and back when at rest. This figure is more like 1 minute to the brain and back. This means that any build up of CO2 in the body would still take 2.5-5 mins to get from the legs to the lungs and once the lungs are resisting the offload of CO2 from the blood, the CO2 in the arterial blood will increase and trigger the chemoreceptors in the aorta, carotid sinuses and the medulla. I think that some simple modelling of blood flow and chemistry would go nicely here - although I don't have the time at the moment. I will certainly give it a go soon.
The thing that has been bugging me for some time about all our theories of blood acidity and O2 saturation etc is that we seem to be relying on qualitative concepts i.e. acidic blood offloads O2 better (and is worse at picking it up, don't forget). The problem with that is that it isn't refernced to the actual figures that the effect curve gives and relating these to the figures that is seen in the blood during a breathold. I know Eric and others have done some great stuff with a lot of instruments (oximeters etc) and gained some proper data. I think we could use that data more effectively if we feed it into some model.
I think this idea of Eric's is great, but I'm still not convinced that we are applying the concept correctly to human physiology. A spot of modelling might go nicely here. I don't have time to do it at present, but may well do so in a week or two. I think we could have much to gain.
Eric - I'll email you to discuss models etc.
