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Re: CO2 transport
On Thu, 13 Jan 2000, Rebecca Allbritton wrote:
> 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.
I don't know what active transport mechanism might work across the
membranes in a fish's gills, but I did read into CO2 transport in blood -
human blood. I suppose that transport in fish is at least somewhat
According to my wife's pathophysiology text (Pathophysiology by McCance
and Huether) CO2 in human venus blood is carried as: 10% dissolved CO2,
60% bicarbonate, and 30% carbamino compounds. Carbamino compounds are
complexes formed between CO2 and blood proteins - particularly hemoglobin.
As near as I can tell the CO2 and the oxygen don't compete directly for
the same sites in the hemoglobin; oxygen is carried by the heme, while CO2
is bound to the protein. Despite the lack of direct competition, there is
an antithetic relationship between hemoglobin's oxygen content and its
ability to transport CO2. The way the text is worded it appears to be the
oxygen content of hemoglobin that controls it's ability to transport CO2,
not the other way around.
I'm not entirely sure what that means to fish physiology. The
predominance of CO2 transport as dissolved CO2 and bicarbonate reflects
the high solubility of CO2 and the inevitable equilibrium between CO2 and
bicarbonate. Probably that mechanism will carry over to fish. Let's
suppose that fish hemoglobin carries CO2 and that oxygen transport can be
suppressed by CO2 (the reverse of the mechanism in humans?). If that's
true then I would expect that high CO2 levels might impair fish's ability
to breath. It would be a partial impairment and it probably wouldn't
suffocate the fish.
That's nearly pure speculation, but it leads me to suspect that the fish
may show effects from acidosis before they show any symptoms of
suffocation from high CO2. Fish Diseases by Mark Dulin describes the
symptoms caused when the water is too acidic (mostly hyperactivity, but it
can include damage to the gills followed by respiratory distress). He
doesn't describe symptoms of acidosis caused by high CO2 in the fish's
bloodstream. The symptoms described by Ghadially -- "they appear drowsy,
rest on the bottom and rock from side to side" -- may be the normal
symptoms of subacute respiratory acidosis in fish.