The Henderson-Hasselbalch equation expressed in terms of carbonic acid

THE HENDERSON-HASSELBALCH EQUATION EXPRESSED IN TERMS OF CARBONIC ACID 

The chemical equation which is the basis of acid base balance, i.e. the combination of carbon dioxide and water to yield hydrogen ions and bicarbonate ions, is 

For any weak acid, the chemical reaction of its dissociation into hydrogen ions and base is written  

In conformity with this convention, the basic chemical reaction in acid-base physiology is to be written  

This gives the Henderson-Hasselbalch equation in the form shown in Table 5.5C. Since the concentration of H2CO3 is so much less than that of CO2, the pK is far below the physiological pH the pK for this reaction is actually 3.6. More commonly, the chemical reaction  

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