H4 K

.E.qu.atlo.n,,,,(,2.,). demonstrates that a buffer system's efficiency is determined by how close its pKA is to physiologic pH. Buffer systems that are effective at physiologic pH

include hemoglobin, phosphate, proteins, and bicarbonate (Fig..,,2l-l).l One can consider the hydrogen ion concentration to be the result of all physiologic buffers acting on the common pool of hydrogen ions.

FIG. 21-1. Schematic representation of hydrogen ion homeostasis.

If the base 10 logarithm of each term in Eq (2) is taken and the resulting equation is applied to the bicarbonate/carbonic acid buffer system, the familiar

Henderson-Hasselbalch equation—Eq (3)—results.

This equation demonstrates the interrelationship between carbonic acid, bicarbonate, and pH; any two of these determine the third.

The clinical utility of the Henderson-Hasselbalch equation, however, is limited. The values of each term in Eq (3) except pH are not convenient to measure. The bicarbonate concentration is approximated by the total CO2,* but carbonic acid must be calculated from the Pco2 and the solubility constant of CO2 in blood. This requires substitution of the pKA with a modified value. Finally, the logarithm of all terms must be calculated.

However, if all constants are inserted into the Henderson-Hasselbalch equation and the antilog of all its terms is taken, the resulting simplified equation— Eq (4)—is of great clinical utility.2

PCQz

The Kassirer-Bleich equation may be used to calculate the concentration of any component of the bicarbonate buffer system provided that the concentrations of the other two components are known. It therefore allows clinicians to determine, for example, what the pH must be when the Pco2 and the bicarbonate concentration are known. Performing such a calculation using values reported by the laboratory permits the clinician to check the internal consistency of the reported data. (In fact, hospital laboratories do not measure [HCO3-] but calculate the number based on pH and Pco2 measurements.)

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