Question: If you are doing a CESA from say 60', does the expanding lung air do anything good for you--ei. getting more 02 to your tissues? My guess would be no, since when the air expands is also becomes "thinner". Perhaps just
Think it through.
Part I: What's going on with the air in the lungs
Just to make up some numbers, let's say that at sea level your lungs have 1 million molecules of oxygen in them with each normal breath. If you are at 20 meters/66 feet of seawater, that same volume of lungs will have 3 million molecules of oxygen. As you ascend and exhale, you will still have more molecules of oxygen in your lungs than you would breathing normally at the surface until maybe the very end, assuming you maintain a normal lung volume throughout the ascent.
Part II: What's going on with blood and body
Your blood circulates throughout the body, passing regularly through the lungs where it comes into contact with the air through the thin walls of the alveoli. As it goes through the body, it carries oxygen, chiefly through the hemoglobin. The blood volume remains the same throughout the experience. The amount of hemoglobin remains the same. The ability of the hemoglobin to carry remains the same. In normal breathing on the surface, we only use a fraction of the oxygen we inhale--the rest is exhaled. We therefore have more than we need. When we are diving, each lungful contains even more oxygen than we need, and most of it is just exhaled. During a CESA, the blood continues to pass through the excess oxygen in the alveoli, just as it did before the CESA.
Part III: What about oxygen already in the tissues?
Our sadly departed diving physician, Lynne Flaherty (TSandM) used to explain that the body has enough oxygen in its tissues to sustain consciousness for 1 to 1.5 minutes, even with no additional oxygen coming from the blood.
Part IV: Why do I have that panicky urge to breathe--doesn't that mean I need oxygen badly?
Nope. Although the body does have a signal when it needs oxygen, it is barely perceptible, and most people have never even experienced it. That panicky urge to breathe is created by the buildup of carbon dioxide, which is in turn caused by our failure to exhale rather than our failure to inhale.