The bicarbonate ion concentration may be calculated from the total CO2 and PCO2 blood levels by using which formula?

The bicarbonate ion concentration may be calculated from the total CO2 and PCO2 blood levels by using which formula?

• A. 0.03 x (PCO2 - total CO2)
• B. (total CO2 + 0.03) x PCO2
• C. 0.03 x (total CO2 - PO2)
• D. total CO2 - (0.03 x PCO2)

Answer: (D)

The key idea is that total CO2 in blood is made up of bicarbonate (HCO3−) plus dissolved CO2. The portion that is dissolved depends on how much CO2 pressure is present, quantified by PCO2, with a solubility factor of about 0.03. So dissolved CO2 ≈ 0.03 × PCO2. Therefore, bicarbonate = total CO2 − dissolved CO2 = total CO2 − (0.03 × PCO2). This is the correct relationship. The other options mix up the operations, use the wrong variable (PO2), or apply the coefficient incorrectly, which doesn’t reflect how dissolved CO2 relates to PCO2.

The key idea is that total CO2 in blood is made up of bicarbonate (HCO3−) plus dissolved CO2. The portion that is dissolved depends on how much CO2 pressure is present, quantified by PCO2, with a solubility factor of about 0.03. So dissolved CO2 ≈ 0.03 × PCO2. Therefore, bicarbonate = total CO2 − dissolved CO2 = total CO2 − (0.03 × PCO2). This is the correct relationship. The other options mix up the operations, use the wrong variable (PO2), or apply the coefficient incorrectly, which doesn’t reflect how dissolved CO2 relates to PCO2.