Henderson-Hasselbalch Equation

The Henderson-Hasselbalch (H-H) equation illustrates the incredibly important role of the bicarbonate buffer system comprised of the partial pressure of carbon dioxide (PaCO2, primarily regulated by the lungs) and bicarbonate (HCO3, primarily regulated by the kidneys). It represents the most extensive buffer system in the extracellular space.

Based on the equation, as the bicarbonate concentration increases, so does the pH (more basic/alkaline), and as the pCO2 increases, the pH will decrease (more acidotic). This relationship is rooted in the carbonic anhydrase equilibrium: [H2O] + [CO2] ⇆ [H2CO3] ⇆ [HCO3] + [H+]. In the equation, ‘6.1’ represents the dissociation constant of carbonic acid, and ‘0.03’ is the solubility constant of carbon dioxide. Plug in the “normal” values of 24 mmol/L and 40 mmHg for HCO3 and pCO2, respectively, and you’ll arrive at the “normal” pH of 7.4.

Remember that the pH and pCO2 are directly measured on an arterial blood gas (ABG), but the bicarbonate value on that same ABG is CALCULATED using the H-H equation. You’d need a BMP or CMP to obtain a directly measured bicarbonate value.

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