Carriage of carbon dioxide


3 ways that carbon dioxide is transported back to the lungs:

  • 5% is dissolved in the blood plasma as CO2.
  • 10% is transported as carbinohaemoglobin (haemoglobin combined with carbon dioxide)
  • 85% as hydrogencarbonate ions 


1. carbon dioxide diffuses into the blood, some will diffuse into a red blood cell.

2. in the erythrocyte CO2 combines with H2O forming carbonic acid, H2CO3, under the influence of the carbonic andydrase catalyst.

3. the H2CO3 then dissociates, releasing H+ ions ans CO3- (hydrogencarbonate ions), the hydrogencarbonate ions diffuse out of the red blood cell into the blood plasma.

4. chloride ions, Cl-, diffuse into the red blood cell to balance the charge in a process called the chloride shift.

5. to stop the RBC becoming acidic the hydrogen ions are taken up by haemoglobin, Hb, forming haemoglobinic acid, HHb. In


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