Carbon dioxide is transported in the blood in three forms:
Approximately 90% as bicarbonate
Approximately 5% as carbamino compounds
Approximately 5% in physical solution

Carbon dioxide transport from tissue to red blood cell
(1)
Carbon dioxide passes from tissue to blood down its partial pressure gradient.
(2)
The carbon dioxide combines with water to form carbonic acid.
(3)
Carbonic acid dissociates into protons and bicarbonate ions due to carbonic acid being a weak acid.
(4)
As bicarbonate ions are produced a concentration gradient is set up between the red blood cell and the blood plasma. As a result bicarbonate ions diffuse out of the cell and into the plasma.
(5)
Bicarbonate ions exiting the cell results in the red blood cell being positively charged relative to the blood plasma. Therefore chloride ions enter the cell from the plasma to maintain electrical neutrality. This exchange is called the chloride shift.
(6)
Protons are produced from dissociation of carbonic acid and also of carbamino compounds. These protons cannot exit the cell due to the redcell membrane being impermeable to protons, the imidazole groups of haemoglobin therefore act as a buffer to remove the protons.

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