In what form is CO2 primarily transported in blood?

Carbon dioxide (CO2) is primarily transported in the blood as bicarbonate ions (HCO3-). The process begins when CO2 diffuses into red blood cells, where it is enzymatically converted to carbonic acid (H2CO3) by the action of the enzyme carbonic anhydrase. Carbonic acid is unstable and quickly dissociates into bicarbonate and hydrogen ions. The bicarbonate ions then diffuse out of the red blood cells into the plasma, where they travel to the lungs for exhalation.

This bicarbonate formation is critical for maintaining acid-base balance in the body and plays a significant role in the respiratory process of gas exchange. While CO2 can be transported in other forms, such as dissolved CO2 and as carbamino compounds (which are formed when CO2 binds to hemoglobin and other proteins), these forms represent a much smaller fraction of CO2 transport compared to bicarbonate. Carbonic acid itself is not a major transport form since it exists transiently before dissociating into bicarbonate and hydrogen ions. Therefore, the predominant form of CO2 in the blood is bicarbonate ions.