8.2.3 Plasma & Carbon Dioxide

• Waste carbon dioxide produced during respiration diffuses from the tissues into the blood 
• This waste carbon dioxide is transported around the body in different ways
  • In the blood plasma in the form of hydrogen carbonate ions (HCO₃^-); around 85 % of carbon dioxide is transported in this way
  • Around 5 % of carbon dioxide dissolves directly in the blood plasma
  • Bound to haemoglobin as carbaminohaemoglobin; this accounts for around 10 % of carbon dioxide transport in the blood

Carbon dioxide in the plasma

• Carbon dioxide released as a waste product from respiring cells diffuses into the cytoplasm of red blood cells
• Inside red blood cells, carbon dioxide combines with water to form H₂CO₃

CO₂ + H₂O  ⇌  H₂CO₃

• • Red blood cells contain the enzyme carbonic anhydrase which catalyses the reaction between carbon dioxide and water
  • Without carbonic anhydrase this reaction proceeds very slowly
  • The plasma contains very little carbonic anhydrase hence H₂CO₃ forms more slowly in plasma than in the cytoplasm of red blood cells
• Carbonic acid dissociates readily into hydrogen ions (H⁺) and hydrogen carbonate ions (HCO₃^-)

H₂CO₃  ⇌  HCO₃^– + H⁺

• Hydrogen ions can combine with haemoglobin, forming haemoglobinic acid and preventing the H⁺ ions from lowering the pH of the red blood cell
  • Haemoglobin is said to act as a buffer in this situation
• The hydrogen carbonate ions diffuse out of the red blood cells into the plasma to be transported in solution

Hydrogen carbonate ions form inside red blood cells before diffusing into the blood plasma