Mat,
Some how I missed your question earlier. Why Tanya had you exhale was to decrease your buoyancy with still plenty of O2 left to avoid black out. Actually exhale diving is common as both a training tool to induce the dive reflex, practice of deep equalization in a swimming pool, and as way to dive. Sebastian Murat has pioneered this technique in research, world record depths, training, and instruction and the db community is privileged to have his input on this forum.
We had a scientist and Sebastian arguing and then conversing on the forum a year or two and I think the O2 in the blood and tissue of a person on a full breath of air accounted for something like 54% of a person’s O2 when a person hold's their breath. Therefore you should have no problem holding your breath and diving to 25 feet on just the O2 in your blood and tissue. Also the problem of lungs expanding toward the surface on an ascent and drawing O2 out of the blood is reduced.
There are also several O2 conservation adaptations that take place when a person is submersed in water and descends with increase water pressure. These adaptations take place quicker with less air in the lungs. which can increase a person’s time and depth under the water to much more than 54% of their ability with a full breath.
Actually the weight one needs in freediving is as much related to the size of their lungs and the amount they fill them as it is to their fat percent. People marvel at how little weight I use, but in reality it’s probably because of my small lungs more than my fat percent, which isn’t that low.
don
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