That is, when several buffers are present together in the same solution, they are all exposed to the same hydrogen ion activity.
Hence, the pK of each buffer will dictate the ratio of the concentrations of its base and weak acid forms at the given pH, in accordance with the Henderson-Hasselbalch equation.
The isohydric principle has special relevance to in vivo biochemistry where multiple acid/ base pairs are in solution.
That is, in vivo, knowing the concentration of pCO2 (weak acid) and bicarbonate (conjugate base) and the pKa of that buffer system, the pH can be calculated regardless of the presence of other contributing buffers.
The clinical relevance is that arterial blood gas often directly measures the CO2 levels and the pH, but the bicarbonate levels are then calculated from that information—without regard to other buffers present[1]