Why do people who are holding their breath underwater blow bubbles, or let air escape from their mouths?

Question

I see this a lot, especially in the movies, but since I was a child I never understood why. I realize that when you normally breathe in and out, you are not absorbing all the oxygen from the air you breathe. Thus, shouldn't a person who wants to stay underwater for as long as possible not exhale underwater, as to fully "absorb" all the oxygen from the air in his lungs?

Answer 1

The only reason to let air out is 2 either get rid of carbon dioxide, or to keep air from getting in your nose.

Answer 2

The trigger in your brain that lets you know to breath is regulated by the carbon dioxide concentration in your blood. Thus by releasing air it pushes out Carbon dioxide. The best is to release little air continuously over a longer period of time.

Answer 3

The purpose is to release carbon dioxide accumulated inside your body. To blow out buble thru nose is to avoid water from getting inside your nose which can cause choking.