No, it goes into your lungs and provides buoyancy for as long as you hold that breath. Some of the oxygen gets taken into your bloodstream, where it no longer provides buoyancy, but that's compensated by an (approximately) equal amount of CO2 that you exhale. But unless you blow it back in the bag, it ceases to provide you any buoyancy because it goes back into the atmosphere. (And if you do blow it back into the bag, your next breath will contain rather a lot of CO2...and the next one after that, even more.)