Scubadiving - What's the Difference?

If you were to take a breath from a pressurized breathing system of some sort, you could then open your mouth point up and start swimming straight up and exhale. As you swim up, the pressure around you goes down, and so that volume of air that you have in your lungs will grow. And so as it expands more, there's more oxygen inside that compressed volume. Semake sense? You're saying you would have oxygen all the way up. Yes, at least i did.