CO2 Transport

• Carbon dioxide is carried in solution and bound to haemoglobin in the blood
• Carbon dioxide is transformed in red blood cells into hydrogen carbonate ions
• Carbon dioxide is transported between the lungs and the tissues by one of three mechanisms:
• 1. Some is bound to haemoglobin to form HbCO2 (carbon dioxide binds to the globin and doesn’t compete with O2 binding)
• 2. A very small fraction gets dissolved in water and is carried in solution (~5% - carbon dioxide dissolves poorly in water)
• 3. The majority (~75%) diffuses into the erythrocyte and gets converted into carbonic acid
• Transport as Carbonic Acid:

  • CO2 combines with water to form carbonic acid in the erythrocyte (reaction catalysed by carbonic anhydrase)
  • Carbonic acid (H2CO3) dissociates to form hydrogen ions (H+) and bicarbonate (HCO3–)
  • Bicarbonate is pumped out of the cell in exchange with chloride ions to ensure the erythrocyte remains uncharged
  • Bicarbonate in the blood plasma combines with sodium to form sodium bicarbonate (NaHCO3), which travels to the lungs
  • Hydrogen ions within the erythrocyte make the environment less alkaline, causing haemoglobin to release its oxygen
  • Haemoglobin absorbs the H+ ions and acts as a buffer to maintain intracellular pH
  • When the red blood cell reaches the lungs, bicarbonate is pumped back into the cell and the process is reversed