Explanation

Transportation of oxygen from lungs to body tissues and of $CO_{2}$ from tissues to the lungs is vital role of blood. Transport of carbon dioxide : Most of the $CO_{2}$ that dissolves in blood plasma reacts with water to form carbonic acid : $CO_{2} + \ H_{2}O\ \rightarrow H_{2}CO_{3}$ An enzyme carbonic anhydrase present in RBCs, which accelerates the carbonic acid formation about 5000 times. About 70% of the $CO_{2}$ received by blood from the tissue immediately enter into RBCs and hydrated to carbonic acid. All carbonic acid of RBCs dissociates into hydrogen and bicarbonate ions $\left( H^{+}\ and\ HCO_{3}^{-} \right).$ The $H^{+}$ mostly combine with haemoglobin for keeping the pH of blood (7.4) in steady state, whereas the bicarbonate ion diffuse from RBCs into the plasma. To maintain electrostatic neutrality of plasma many chloride ions in turn diffuse from plasma into RBCs. This is termed chloride or Hamburger shift.