Hydrogen Carbonate Ions

• Most carbon dioxide diffuses from the plasma into red blood cells. Inside red blood cells carbon dioxide combines with water to form carbonic acid, H2CO3

  CO2 + H2O  ⇌  H2CO3
• Red blood cells contain the enzyme carbonic anhydrase which catalyses this reaction. Plasma contains very little carbonic anhydrase. Without carbonic anhydrase this reaction proceeds very slowly, therefore H2CO3 forms more slowly in plasma than in the cytoplasm of red blood cells.
• Carbonic acid dissociates readily into hydrogen ions (H+) and hydrogen carbonate ions (HCO3-):

  H2CO3  ⇌  HCO3– + H+
• The increase in hydrogen ions causes oxyhaemoglobin to unload its oxygen. The hydrogen ions can then combine with haemoglobin, forming haemoglobinic acid.  This prevents the H+ ions from lowering the pH of the red blood cell, so haemoglobin is said to act as a buffer in this situation. The hydrogen carbonate ions diffuse out of the red blood cell into the blood plasma where they are transported in solution.
• The chloride shift is the movement of chloride ions into red blood cells that occurs when hydrogen carbonate ions are formed from the dissociation of carbonic acid:

   H2CO3  ⇌  HCO3– + H+
• The hydrogen carbonate ions are transported out of red blood cells via a transport protein in the membrane. To prevent an electrical imbalance, negatively charged chloride ions are transported into the red blood cells via the same transport protein. If this did not happen, red blood cells would become positively charged as a result of a build up of hydrogen ions formed from the dissociation of carbonic acid.