Abbo

The number of molecules of oxygen your body needs is a straight function of work rate. So, for a low level of work in a normal dive your body will metabolise around 0.8l/min of free oxygen. That is independent of the PO2. As the oxygen partial pressure rises, reactions can occur that would not occur at lower PO2s, such as the creation of free radicals like the superoxide radical. These cause all kinds of damage to your body, ultimately causing you to breakdance with an ox tox hit. The increase in oxygen being absorbed by your body is negligible, however.

You mention the PN2: at 15m, the PN2 is 1.8 with a set point of 0.7 and 1.1 with a setpoint of 1.4. The effect of narcosis can be to increase breathing rates, but I doubt that means the body is actually absorbing appreciably more exygen - merely hyperventilating, and hence increasing the risk of building up CO2, increasing the risk of an ox tox hit. Still, narcosis does not really set in till the PN2 gets to around 4.

Obviously, Henry's Law says that the rate a liquid absorbs a gas is a function of the partial pressure of the gas, so ongassing of nitrogen will be faster with the lower set point.