What is the term that describes the sum of carbonic acid and bicarbonate in plasma?

• A. Total CO2
• B. Standard bicarbonate
• C. Buffer base
• D. Base excess

Answer: (A)

In plasma, carbon dioxide exists mainly as bicarbonate, with carbonic acid in rapid equilibrium with dissolved CO2. The total amount of these CO2 species—the sum of carbonic acid and bicarbonate—is described as Total CO2. This reflects all nonvolatile CO2 present in the plasma, not just one form.

The other terms refer to different concepts: Standard bicarbonate is the bicarbonate concentration at a reference pH of 7.40; Buffer base represents the buffering capacity from bases other than CO2; Base excess indicates how much strong base or acid would be needed to shift the pH to 7.40. None of these capture the combined carbonic acid plus bicarbonate content like Total CO2 does.