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J Shepherd writes:
> 
> 	I was under the impression that a small percentage of the
> population do use O2 levels rather than CO2 levels to breathe. I
> remember nothing about whether it is caused by a random event during
> your development; a response to continual high CO2 levels; or a
> mutation.

The middle option.  Chronic Obstructive Pulmonary Disease (COPD) is
one such disease which causes the CO2 drive to be ignored, because the
body simply cannot ventilate the lungs well enough to keep CO2 down
below the chemoreceptors trigger.  This condition leads to
desensitization and reliance on the O2 level to trigger breathing,
usually around 50mmHg pO2.

An interesting aside: the level of pCO2 at the breath-hold breaking
point has been shown to be higher in an exercising subject than in a
resting subject.  This means that an exercising diver can further
extend their bottom time, while using more O2, thus making exercising
divers more likely to achieve shallow-water blackout.

Cheers,

David Story                        NAUI AI Z9588, PADI DM 43922, EMT
story@be*.wp*.sg*.co*		   Oxygen is a drug in California.

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