The three forms of CO2 in blood
Carbon dioxide produced by tissues is carried back to the lungs in three forms. The majority, around 70 percent, travels as bicarbonate ions dissolved in plasma. About 20 to 23 percent binds to amino groups on hemoglobin and other proteins as carbamino compounds. The remaining 7 to 10 percent stays physically dissolved in the plasma.
Most of the bicarbonate is generated inside red blood cells, where the enzyme carbonic anhydrase rapidly converts CO2 and water into carbonic acid, which dissociates into bicarbonate and a hydrogen ion.
The chloride shift and the Bohr and Haldane effects
As bicarbonate accumulates in the red cell it diffuses out into plasma, and chloride ions move in to maintain electrical balance, the chloride or Hamburger shift. The freed hydrogen ions are buffered by hemoglobin.
Two coupled effects link oxygen and CO2 carriage. The Haldane effect: deoxygenated hemoglobin binds CO2 and protons more readily, so the tissues load CO2 efficiently. The Bohr effect: higher CO2 and acidity make hemoglobin release oxygen more easily where it is needed. In the lungs these run in reverse to unload CO2 and pick up oxygen.