How is CO2 transported in the blood?

• A. Mostly as bicarbonate in plasma; some bound to hemoglobin and dissolved CO2
• B. Primarily dissolved CO2
• C. Bound to albumin
• D. Carried by platelets

Answer: (A)

The main idea is how carbon dioxide is carried in the blood. Most CO2 is transported as bicarbonate ions in plasma. In red blood cells, CO2 reacts with water under the action of carbonic anhydrase to form carbonic acid, which quickly dissociates into bicarbonate (HCO3−) and hydrogen ions. The bicarbonate is then traded with chloride across the red cell membrane (the chloride shift) to enter the plasma, making bicarbonate the dominant transport form. A smaller portion of CO2 is carried dissolved in plasma, and another portion binds to hemoglobin to form carbaminohemoglobin. This combination—bicarbonate in plasma as the major form, with some dissolved CO2 and some bound to hemoglobin—explains the primary modes of CO2 transport. The other options aren’t correct because CO2 is not primarily transported dissolved, it is not mainly bound to albumin, and platelets do not carry CO2 for transport. In the lungs, the reverse reactions release CO2 for exhalation.