> What you really need is a biological diode - a permeable membrane that
> is more permeable one way than the other for a given target gas.

The lung / body system achieves that in another way :-).
It makes a '"diode"  by altering the transiting item.

When an Oxygen molecule enters the lung the system attaches a carbon to it
The lung membrane passes CO2 at about 100 times the rate that it
passes O2. So CO2 exits rapidly and easily and so tends to a lower %
pp automatically.

When lung membrane decays it usually runs out of O2 sufficiency first
and you try to die from lack of oxygenation. If you give supplemental
)2 the system keeps decaying until CO2 transport then becomes a
problem and then you start getting acidification problems when the CO2
isn't removed rapidly enough.


          R
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