Which of the following describes carbon dioxide transport in the blood?

• A. Oxygen dissolved in plasma; carbon dioxide bound to hemoglobin.
• B. CO2 is dissolved as CO2, carbaminohemoglobin, or bicarbonate ions.
• C. CO2 transported only as dissolved CO2 gas.
• D. CO2 transport relies solely on bicarbonate in urine.

Answer: (B)

CO2 is carried in the blood in three main forms: dissolved in plasma, bound to hemoglobin as carbaminohemoglobin, and as bicarbonate ions formed inside red blood cells. In tissues, CO2 diffuses into red blood cells, where the enzyme carbonic anhydrase converts CO2 and water to carbonic acid, which quickly dissociates into bicarbonate and hydrogen ions. The bicarbonate exits the cell in exchange for chloride, so most of the CO2 is transported as bicarbonate in plasma. A portion also binds to amino groups on hemoglobin to form carbaminohemoglobin. Some CO2 remains dissolved in plasma. In the lungs, the process reverses: bicarbonate re-enters red blood cells and combines with hydrogen to form CO2 and water, which is then exhaled, and CO2 bound to hemoglobin is released as oxygen binds. This multi-form transport accurately reflects how CO2 is moved from tissues to the lungs.