7th February 2024

Carbon dioxide

Carbon dioxide is carried in blood in various forms:

• Bicarbonate ion (HCO3-) (90%)
  • Carbonic anhydrase enzymes found in abundance in red blood cell (RBC) cytoplasm catalyse the reaction CO2 + H2O ⇋ H2CO3 ⇋ H+ + HCO3-
  • H+ cannot cross the RBC membrane. H+ binds to Hb, forming HHb.
  • HCO3- in RBC is transported out of the cell down its electrochemical gradient in exchange with plasma Cl- to maintain the electrical neutrality of the cell. This process is referred to as ‘chloride shift.’
• Carbaminohaemoglobin (5%)
  • CO2 binds to haemoglobin to form carbaminohaemoglobin (do not confuse this with carboxyhaemoglobin, COHb, which is carbon monoxide bound to Hb)
  • Deoxyhaemoglobin binds more readily to carbon dioxide (Haldane effect)
• Dissolved as carbonic acid in plasma (5%)