
PCO_{2} 40

×  K 24

=  [H^{+}] 40
nanoEq/L

×  HCO_{3}^{} 24


pH 7.4

SBE 0


As you adjust the PCO_{2}, the SBE is held constant; when adjusting the [H^{+}], the PCO_{2} is held constant.
Henderson's equation quantitates the relationship between [H^{+}], [HCO_{3}^{}], and PCO_{2}. In other words it performs precisely the same function as the more intimidating HendersonHasselbalch Equation. It does it, however, without using negative logarithms. It is, accordingly, much easier to understand:
The Interactive Henderson Equation: Use the Equation above to vary the values and observe the automatic interpretation.
The year 1908 was a good year for acidbase balance  thanks to Henderson. In that year he appreciated the buffering power of CO_{2} and went on to apply the law of mass action to produce his simple formula which, rearranged, looks like this:
Simplification is possible because [H_{2}O] remains constant and physicians are much more familiar with PCO_{2}. This gives us the version used in the Interactive Equation:
Easy to understand  most school children could grasp its meaning  which probably means that even medical students could, too (!) The simplicity, however, also guaranteed that it was of little "interest". Next year, 1909, was the start of a downhill slide: Sorensen introduced the negative logarithm (pH). This paved the way for Hasselbalch. In 1916 he combined Henderson's excellent equation with Sorensen's pH to produce the dreaded HendersonHasselbalch equation.
Acidbase balance became instantly incomprehensible and, therefore, fascinating to teach. The new equation contained no extra information for physicians; it solved no medical problem; and it added nothing to our sum of acidbase physiology except obfuscation and unnecessary exam questions.
Three generations of doctors have endured the HendersonHasselbalch equation. It's time to make a stand. Enough! Play with the Modified Henderson Equation. Appreciate that it is the whole truth. If you must convert between pH and [H^{+}], then do so, but do it in two simple steps. This way you may understand what you are doing:
AcidBase Tutorial Alan W. Grogono 
Copyright Oct 2016. All Rights Reserved 