good question, hopefully someone who knows max co2 level can answer

here's a guess along the way:
if average xsection is abt 200sq in and bulkhd to blkhd is 60in get abt 7cu ft. minus body and equip say 4cu ft left over.

one max breath is abt 4.5litre=.17cuft

so if we just breath in and exhale into the water and let waterlevel rise in yak in perfect conditions w/ just enough water coming in would get abt 24 full breaths.

my guess in the closed system is that you could go for 1/3 of this, say 8 breaths. could anyone answer better?

what percentage of co2 do we breath out, what ultimate percentage would stop us from trying one more roll?

would have to purge all the 'air' to do it again.

(shawn, i think embolism is only a danger from greater depths than just upside down and only if pressurized air-more capacity that lung can carry is breathed in - like can free divers embolize if get the air from air pressure returned to ??)
((he he and what if our bottom exiting footpumps now could reverse? constant air supply??? espec if electric?))