carbon dioxide transport by haemoglobin
Last reviewed 01/2018
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