I more meant because I'm a woman, not just the air in my lungs will make buoyant lol. I do know about the CO2 build up stuff though, I really feel it in my chest after about 2 minutes of being underwater.I find that having air in my lungs buoys me up. I can't even go below the surface without fins unless I let air out. The feeling of "needing to breathe" is actually caused by CO2 buildup, not by using up oxygen. My underwater time is so short that it makes no real difference letting the air out. If I trained for longer submergence, I might learn differently.