The Henderson–Hasselbalch equation, when applied to the carbonic acid-bicarbonate buffer system in the extracellular fluids, states that:
where:
*  is the negative logarithm (or cologarithm) of molar concentration of hydrogen ions in the extracellular fluid.
*  is the cologarithm of the acid dissociation constant of carbonic acid. It is equal to 6.1.
*  is the molar concentration of bicarbonate in the blood plasma.
*  is the molar concentration of carbonic acid in the extracellular fluid.
However, since the carbonic acid concentration is directly proportional to the partial pressure of carbon dioxide () in the extracellular fluid, the equation can be rewritten as follows:
where:
*  is the negative logarithm of molar concentration of hydrogen ions in the extracellular fluid.
*  is the molar concentration of bicarbonate in the plasma.
*  is the partial pressure of carbon dioxide in the blood plasma.
The pH of the extracellular fluids can thus be controlled by the regulation of  and the other metabolic acids.