Carbon dioxide transport by haemoglobin

Carbon dioxide is produced by the tissues and diffuses into the plasma. In the plasma:

• a small proportion becomes bound to plasma proteins forming carbamino compounds
  
  
• a small proportion reacts slowly to form carbonic acid which dissociates to form a hydrogen ion and bicarbonate
  
  
• largest proportion enters the red cell where it rapidly forms carbonic acid:
  • this reaction is catalysed by carbonic anhydrase
  • within the red cell, the carbonic acid dissociates and the hydrogen ion combines with the reduced haemoglobin and the bicarbonate diffuses out of the cell
  • the loss of anionic bicarbonate from the cell is compensated (maintaining electrical neutrality) by an influx of chloride ions (anions) - the so-called 'chloride shift'
    
    
• 5-10% of carbon dioxide is transported via direct binding of carbon dioxide to haemoglobin (carbaminohaemoglobin)
  • production of this compound is increased in conditions of hypoxia and reduced when the haemoglobin is oxygenated