Carriage of carbon dioxide

How is carbon dioxide carried

• 5% is dissolved in plasma

• 10% as carbaminohaemoglobin, forms when CO₂ combines with haemoglobin

• 85% as hydrogen carbonate ions, HCO3,-

How do the hydrogen carbonate ions form

1. CO₂ diffuses from plasma to RBC and in RBC combines with H₂O to form H₂CO₃ (carbonic acid) CO₂ + H₂O  ⇌  H₂CO₃

2. Carbonic acid dissociates readily into H⁺ and HCO₃^- ions H₂CO₃  ⇌  HCO₃^– + H⁺

Carbonic anhydrase catalyses this reaction

What is the chloride shift

Carbonate ions, which are negatively charged, are transported out of RBC via transport proteins. To prevent an electrical imbalance negatively charged chloride ions are transported into RBC via the same transport proteins.

What would happen if there was no chloride shift

There would be a build up of H⁺ ions decreasing the pH of blood which would affect enzyme activity

What is the Bohr shift

This occurs when partial pressure of carbon dioxide in blood is high. H⁺ ions which are formed from carbonic acid change quaternary structure of haemoglobin, so as a result they have a lower affinity for oxygen. This is useful for respiring tissue as oxygen is released by haemoglobin more readily

How does the Bohr shift affect the dissociation curve

Dissociation curve shifts to the right meaning at any given partial pressure of oxygen, the % saturation of haemoglobin is lower at higher levels of CO₂