Carbon Dioxide Transport

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•  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:

  • Some is bound to haemoglobin to form HbCO2 (carbon dioxide binds to the globin and so doesn’t compete with O2 binding)
  • A very small fraction gets dissolved in water and is carried in solution (~5% – carbon dioxide dissolves poorly in water)
  • The majority (~75%) diffuses into the erythrocyte and gets converted into carbonic acid

Transport as Carbonic Acid

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

Carbon Dioxide Transport in the Bloodstream

carbon dioxide transport