A buffer is a solution in which pH is maintained at a nearly constant value. It consists of a relatively concentrated solution of a weak acid and its conjugate base, and works best if their concentrations are roughly equal, in which case the hydrogen ion concentration equals the dissociation constant of the acid. When a small amount of a different acid or base is added, the buffer equilibrium shifts so that the pH value hardly changes. One common buffer is acetic acid and sodium acetate. Buffers are used in many chemical and biochemical experiments. Biochemical processes in the body are controlled by natural buffer systems. The chief buffer of the blood and extracellular body fluids is the bicarbonate (H₂CO₃/HCO₃^-) system.