A  buffer  minimize  changes  in  pH  by
   accepting  hydrogen ions from the solution when they are in excess and
   donating hydrogen ions to the solution when they are low.
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   In human blood, dissolved CO[2] forms Carbonic acid (H[2]CO[3]).
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   Carbonic  acid acts as a buffer by dissociating to yield a bicarbonate
   ion (HCO[3]^-) and a hydrogen ion (H^+) in a dynamic equilibrium.