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> A well educated non diver asked me a question yesterday, and caught me flat > footed. If you are at depth, why can't you hold your breath longer? You have > more O2 in your lungs, and CO2 would build up more slowly. Is it CO2 buildup > in the body or brain, not the lungs, that causes the breathing reflex? And > yet hemoglobin can't work with the air stored in the lungs for a sufficient > refresh to suppress the breathing reflex? The breathing reflex is caused by the level of CO2 in the blood. The detection of this is in the brain. Since the same amount of CO2 is created by the body (same amount of work is being done), the bicarbonate levels and acidity of the blood will be the same and you still need to breathe to reduce it. The O2 levels are not a problem because it is the partial pressure that is important here. This is why you can dive using mixed gas with very low percentages of oxygen. And it's also why shallow water black-outs occur (a reduction in the partial pressure of the oxygen). This is a laymans understanding. I am not a physiologist. Corrections or more details are very welcome. Devon
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