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] [acid]

This is the Henderson Hasselbalch equation. [3]

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:

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. [3] The pCO₂ is controlled by the lungs and the HCO₃^– by the kidneys (see the page on bicarbonate control).