Think piece: about gas absorption/physiology

warjarrett

If I dive this week to 45 ft (15m) for 1 hour at fixed 0.7 PO2, then exactly the same dive next week but at 1.4 PO2 (assume I spend very little time ascending or descending), how will my gas consumption of O2 and air/dil compare? In other words, will my body absorb more O2 and/or more dil during either of these 2 dives? Will I have to add or release any more O2 and/or dil during either of these 2 dives? Please explain your answer.

Dsix36

Your PPO2 will be higher and thus causing more absorption, but the actual difference in gas usage will be unnoticeable.

The addidion of gas will remain the same also (unles your workload varies from the two dives)

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warjarrett

I am thinking that O2 absorption will be absolutely identical, because the physiology of oxygen use is a matter of effort expended, NOT concentration of the breathing gas (and, by the way, not depth either). Therefore O2 pressure will be no different for each of the dives. But more N2 WILL be absorbed when its concentration is higher. So the N2 absorption from the dill WILL be different in the 2 cases. For the dive with higher PO2, less air will be used. What do you think?

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.

Simon Mitchell

Hello,

Oxygen consumption is driven by metabolic need, not the PO2 available, so assuming your dives are identical in every way other than the gas changes you specified, oxygen consumption will not change. Nor will you absorb more oxygen. While the arterial PO2 will be higher on the dive with a set point of 1.4, the PO2 in the tissues themselves will not change because the oxygen is metabolised. The hemoglobin may unload a little less oxygen, and the venous hemoglobin may be slightly more saturated with oxygen on the 1.4 dive than the 0.7 dive.

As has been pointed out, absorption of inert gas is driven by the partial pressure, which is higher on the 0.7 dive: therefore you will absorb more nitrogen. However, the total difference in volume absorbed will be sufficiently small that you will not notice it on your dil pressure gauge.

Warm regards,

Simon M