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Re: CO2 transport

> No, no, no -- you've forgotten about active transport, which is one way an
> organism achieves higher concentrations of the desired material (some
> gasses, e.g. O2, & some other molecules) inside the cell than outside, and
> lower concentrations of unwanted materials inside than outside. 

There is no active transport of oxygen.  All cell surfaces are permeable to
O2, even those that are relatively impermeable to water.  Oxygen which
enters an animal across a respiratory surface or into cells must diffuse via
an aqueous solution; the availability of oxygen depends on its concentration
immediately outside the organism.

Ditto for carbon dioxide, although it is a little more complicated because
of the formation of carbonic acid and bicarbonate. 

Source:  Prosser, C.  1991.  Comparative Animal Physiology.  John Wiley, New