Bicarbonate accounts for approximately what percentage of total CO2 in blood?

• A. 70%
• B. 90%
• C. 50%
• D. 100%

Answer: (B)

Most CO2 in blood is carried as bicarbonate. CO2 diffuses into red blood cells and is rapidly converted by carbonic anhydrase to carbonic acid, which dissociates into bicarbonate and hydrogen ions. The bicarbonate is then moved into the plasma in exchange for chloride (the chloride shift), so the major form of circulating CO2 becomes bicarbonate. This mechanism makes bicarbonate account for roughly 90% of total CO2 in blood, with the rest existing as dissolved CO2 and as carbamino compounds such as carbaminohemoglobin.