Question

explain how most carbon dioxide is carried in the blood. ( this question is not asking about which vessels blood flows through).

Answer 1

Ans : The most carbon dioxide is carried in the blood as bicarbonate ions. About 75% of carbon dioxide transport in the red blood cell and 25% in the plasma. This is because plasma lacks an enzyme called carbonic anhydrase which is responsible for rapid inter-conversion of carbon dioxide and water into carbonic acid, protons (H⁺) and bicarbonate (HCO₃^-) ions.

Mechanism : Carbon dioxide and water diffuse freely into the red blood cell and are converted to carbonic acid, a reaction catalyzed by carbonic anhydrase. Carbonic acid is an unstable and therefore immediately dissociates into bicarbonate ions (HCO₃⁻) and hydrogen (H⁺) ions. Therefore, the concentration of both H⁺ and HCO₃^- is increased in the red blood cell. Since carbon dioxide is quickly converted into bicarbonate ions, this reaction allows for the continued uptake of carbon dioxide into the blood down its concentration gradient. HCO₃^- can diffuse out of the red blood cell into plasma whereas H⁺ cannot. In order to maintain electrical neutrality, chloride ions (Cl^-) diffuse into the red blood cell from the plasma as HCO₃^- diffuses out. This is known as the chloride shift (Gibbs–Donnan equilibrium or Hamburger effect). Hydrogen ions are taken up by reduced haemoglobin.