Which factor contributes the most to the serum total CO2?

The serum total CO2 primarily consists of bicarbonate (HCO3-), which is the most significant component in the carbon dioxide transport system in the blood. Approximately 70% of CO2 is transported in the form of bicarbonate ions.

Bicarbonate is formed in red blood cells when carbon dioxide reacts with water, catalyzed by the enzyme carbonic anhydrase, leading to the formation of carbonic acid (H2CO3). This carbonic acid dissociates into hydrogen ions (H+) and bicarbonate ions. This process is critical for maintaining acid-base balance in the body, making bicarbonate a dominant factor in total CO2 concentration measured in serum.

While PCO2 is important in the context of respiratory function and acid-base balance, it represents only a fraction of the total CO2 content in serum. The other options, such as dCO2 (dissolved CO2) and carbonium ions, do not significantly contribute to the serum total CO2 compared to bicarbonate ions. Therefore, bicarbonate is the primary contributor, affirming that option C is the correct answer.