Transportation of carbon dioxide : It is transported in three forms :

1. In solution form : About 7% CO₂ dissolves in plasma and is transported as such.

2. As bicarbonate : About 70% CO₂ from plasma enters into R.B.C. It combines with water to form carbonic acid in the presence of enzyme carbonic anhydrase. Carbonic acid (H₂ CO₃) splits into H⁺ and HCO^-₃

From erythrocytes bicarbonate ions pass into plasma due to which ionic balance between ions in the plasma and erythrocytes is disturbed. To maintain the balance, the chloride ions diffuse into RBC from plasma. This movement of chloride ions is called chloride shift or Hamburger shift. The latter maintains an acid base equilibrium at pH 7.4 for blood and electrical balance between plasma and RBC.

Most of the bicarbonate is carried by plasma and some of it by RBC.

3. As carbamino-haemoglobin : About 23% CO₂ combines with haemoglobin of RBC to form carbamino-haemoglobin. It combines with amino group. The amount of CO₂ which can be carried by haemoglobin is affected by oxygen tension (Haldane effect). The impure blood is carried to heart and then to lungs. In lungs 100 ml of blood releases 3.7 ml of CO₂.