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Hemoglobin buffer system:
The buffer value of non bicarbonate buffers of erythrocyte fluid is about 63 mmol/L.
Hemoglobin accounts for major part (53 mmol/L), with remainder being mainly caused by 2,3-DPG. The imidazole group of hemoglobin is the most important buffering groups.
When oxygenated, H+ ions are liberated from Hb a phenomenon called the Haldane effect.
Similarly release of oxygen by binding to hydrogen is called Bohr Effect.
Protein buffers in blood include haemoglobin (150g/l)
Buffering is by the imidazole group of the histidine residues which has a pKa of about 6.8.
This is suitable for effective buffering at physiological pH.
Haemoglobin is quantitatively about 6 times more important then the plasma proteins as it is present in about twice the concentration and contains about three times the number of histidine residues per molecule.
For example if blood pH changed from 7.5 to 6.5, haemoglobin would buffer 27.5 mmol/l of H+ and total plasma protein buffering would account for only 4.2 mmol/l of H+.
Deoxyhaemoglobin is a more effective buffer than oxyhaemoglobin and this change in buffer capacity contributes about 30% of the Haldane effect. The major factor accounting for the Haldane effect in CO2 transport is the much greater ability of deoxyhaemoglobin to form carbamino compounds.