[popopje]

this question exposes my ignorance of how lungs work, but in a situation where the lungs were in good working order, would carbon dioxide build up in them (to be breathed out)?

[rsanders]

Only until the partial pressure of CO2 was in equilibrium between the volume of air in the lungs and the the body tissues. It's like bailing out a boat -- it only works if you empty the bucket periodically.

[popopje]

could you "breathe" a small volume of something inert in and out, and have a CO2 scrubber?

[teraflop]

Presumably, if your lungs were working well enough for that to be an option, you could just breathe air.