Henderson-Hasselbalch equation

The Henderson-Hasselbalch equation is a specific application of a more general equation used in chemistry; the fact that the hydrogen ion concentration can be derived from a knowledge of the properties of a buffer in a solution and the proportions of the acid and alkali components of the buffer. Thus:

pH = pK + log (HCO3 / CO2)

The equation is used by blood gas machines to predict bicarbonate levels from pH and CO2, which themselves can be directly measured with electrodes. However, a more immediate application for the physician is remembering the consequences of raising or lowering one of the components of the system; thus, for example, raising the carbon dioxide will tend to reduce the pH, or will require a reduction in bicarbonate to maintain the pH.