Anatomy and Physiology II (APII) Milestone 1 Practice Test

Which expression correctly represents the Henderson-Hasselbalch equation for the bicarbonate buffer system, noting that CO2 is converted to carbonic acid in solution?

pH = pKa + log([CO2]/[HCO3-])

pH = pKa + log([HCO3-]/[CO2])

In the bicarbonate buffering system, the Henderson-Hasselbalch expression uses the conjugate base and the weak acid: pH = pKa + log([base]/[acid]). Here the base is bicarbonate (HCO3−) and the acid is carbonic acid (H2CO3), which in solution is effectively represented by dissolved CO2. So the correct form is pH = pKa + log([HCO3−]/[CO2]). The pKa for the carbonic acid/bicarbonate pair is about 6.1 at body temperature, which centers the blood pH around 7.35–7.45 when the ratio of bicarbonate to CO2 is balanced. Swapping the ratio or placing a minus sign would misrepresent how increasing CO2 (more acid) lowers pH, and using pKw isn’t appropriate for this buffer relationship.

pH = pKa - log([HCO3-]/[CO2])

pH = pKw + log([HCO3-]/[CO2])