Henderson Hasselbalch Equation

When the buffer is a weak acid (HA), the pH is related to the ratio of base (A) to undissociated acid (HA) concentration. The acid is constantly in flux between its HA and [H+][A] forms, the balance of which is determined by the equilibrium constant (K). The pH can then be expressed as:

pH = pK + log [conjugate base]

This is the Henderson Hasselbalch equation.

The most important buffer system in the body is the carbonic acid/bicarbonate system due to the physiological control (ie. excretion/retention) of its component parts by the lungs and kidneys, i.e. CO2, H+ and HCO3. This relationship is shown as:

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

For example, when more H+ is added to the system (e.g. in a metabolic acidosis), this shifts the reaction to the left, forming more carbonic acid. This in turn shifts the reaction further to the left forming easily excretable CO2. The pCO2 is controlled by the lungs and the HCO3 by the kidneys (see the page on bicarbonate control).

Note: Henderson Hasselbalch Equation [3]