In the diffusion process CO2 enters the red blood cells and reacts with water with the help of carbonic anhydrase enzyme to produce HCO3. Carbonic acid breaks down into H+ and HCO3- ions. Some HCO3- moves from the red blood cells into the plasma. As much HCO3- enters the blood from the red blood cells as Cl- enters the red blood cells from the plasma. This is called chloride shift or hamburger reaction. The chloride shift reaction is called the Hamburger reaction after the first describer, German physiologist Hartog Jacob Hamburger. This reaction maintains the acid-base balance (pH = 7.4) in the blood.
CO2 + H2O → H2CO3
H2CO3 → H+ + HCO3-
HCO3- ← Exchange → Cl-