I think the lungs aren't strong enough to act as a pressure vessel against the vacuum. Trying to hold it in is like expecting your sensitive tissues to behave the same as a scuba tank.
If you don't hold it in, the air will simply leave your lungs and you have about 12-15 seconds before the remaining oxygen in your blood leaves your body making you unconscious. The lungs, instead of trading CO2 for O2 is now working in reverse in the vacuum, exchanging O2 for... nothing. That amount of time should be enough for you to reattach your O2 hose and open the airlock etc.
I also recall reading somewhere that you may urinate, defecate and projectile vomit simultaneously after a while, however by that time, you may already be unconscious.
If you don't hold it in, the air will simply leave your lungs and you have about 12-15 seconds before the remaining oxygen in your blood leaves your body making you unconscious. The lungs, instead of trading CO2 for O2 is now working in reverse in the vacuum, exchanging O2 for... nothing. That amount of time should be enough for you to reattach your O2 hose and open the airlock etc.
I also recall reading somewhere that you may urinate, defecate and projectile vomit simultaneously after a while, however by that time, you may already be unconscious.