How many times can air be exchanged mouth to mouth underwater?

Question

Or a better question might be, how much oxygen gets absorbed with each breath? And what is the minimum percentage of oxygen that must be present in air to sustain life?

Assuming the person is relaxed with a regular heart rate.

Answer 1

When you exhale, you are not releasing oxygen.  Oxygen is inhaled because our body needs that.  If you are doing CPR underwater, it would be better to bring the body to the surface as the rescuer needs air.  If, like the second respondent infers, you are trying to kiss underwater for a sustained period of time - it won't work.

Answer 2

Through a tube?

Assuming you don't already have a mouthful of water!

Answer 3

That really makes no sense.  What I get from that is: How can air be exchanged while kissing underwater?  Which is a little gross.