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Mnemonic

Carbon Dioxide Transport

A memory aid for the three forms in which carbon dioxide is carried.

Physiology Respiratory Respiratory Respiratory Physiology and Function Tests
Expansion

Bicarbonate about 70 per cent, carbamino about 23 per cent, dissolved about 7 per cent

Expansion

  • Bicarbonate: about 70 per cent. Carbon dioxide enters the red cell, is hydrated by carbonic anhydrase to carbonic acid, which dissociates to bicarbonate and hydrogen ions
  • Carbamino compounds: about 23 per cent. Carbon dioxide binds directly to terminal amine groups on haemoglobin, favoured by deoxygenation
  • Dissolved: about 7 per cent. Much greater than for oxygen, since carbon dioxide is roughly 20 times more soluble

Two supporting mechanisms:

  • The chloride shift (Hamburger phenomenon): bicarbonate leaves the red cell in exchange for chloride, maintaining electrical neutrality and allowing carriage to continue
  • Hydrogen ions generated in the process are buffered by haemoglobin, which is a far better buffer when deoxygenated

The carbon dioxide dissociation curve is much steeper and more linear than the oxygen curve over the physiological range, which is why carbon dioxide is efficiently excreted by increasing ventilation while oxygen loading cannot be similarly improved.