Henderson-Hasselbalch equation:

For carbonic acid/bicarbonate buffer system pK = 6.1.

The normal ratio of bicarbonate to dissolved carbon dioxide in plasma is (25 mmol/L)/(1.25 mmol/L) = 20/1. This equation shows any change in bicarbonate or dissolved CO₂ is accompanied by change in pH.  The numerator characterizes primary metabolic disturbance and is the renal component and denominator characterizes primary respiratory disturbance and is the respiratory component. 

The application of Henderson-Hasselbalch equation to human acid-base physiology can be illustrated by lever-fulcrum (teeter-totter) diagram: