VPB 121: Veterinary Physiology - II (2+1)

Formation of HCO3 by RBCs (Isohydric Transport of CO2)

FORMATION OF HCO₃ BY RBCs

• The CO₂ from tissues enter plasma then into the RBCs. H₂CO₃ formation by the reaction of CO₂ with water is too slow to less of importance in plasma but inside the RBCs, the enzyme carbonic anhydrase catalyses the reaction between CO₂ and H₂O accelerating the rate of about 5,000 fold.
• H₂CO₃ rapidly and spontaneously dissociates into H⁺ and HCO₃. Since an increase in H⁺ concentration is severely detrimental to an organism, a base buffer must be available to remove the H⁺ ions. The Hb provides the buffering, driving the reaction to the right.
• In the tissue Oxy-Hb delivers O₂ and becomes deoxygenated Hb. The deoxygenated Hb is a weaker acid than Oxy-Hb and function as a better buffer readily combines with H⁺ and facilitates the formation of HCO₃ form CO₂.

• Hb is a major buffer in blood that removes free H⁺ from blood and an equal quantity of HCO₃ is left dissolved in fluid. The HCO₃ diffuse out of RBC into the plasma due to concentration gradient by exchanging choloride ions from the plasma across the RBC membrane referred as chloride shift or Hamburger shift.
• The conversion of CO₂ via H₂CO₃ to HCO₃ ion in erythrocytes accounts for 70% for CO₂transport.

• In the lungs, high PO₂ favours diffusion of O₂ into the red cells and oxygenates Hb. On oxygenation HHb becomes Oxy Hb and releases H⁺ ions. This H⁺ ion combine with HCO₃ and drives the reaction to the left.