Buffers help to maintain a consistent pH by adding or removing hydrogen ions as needed.

The slightest change in internal pH can have a devastating impact on the cell. For example, many proteins need an optimal pH to function properly. If that pH changes, even by a tenth of a pH point, a protein may unfold and no longer work. Since nearly all chemical reactions in a cell are assisted by protein enzymes, affecting proteins can cause immediate problems. A shift in pH can also affect transport of substances across cell membranes, and the production of new tissues. Human blood is an example of a solution that must maintain a constant pH. The normal blood pH is approximately 7.4. Any change of more than 0.4 (down to pH 7.0 or up to pH 7.8) will cause a person to become gravely ill. However, cells and bodies are equipped to mediate the effects of a constant input of acids and bases that can change pH.

To compensate for changes in pH, living cells use buffers. A buffer is a solution that resists a change in pH when acid or base is added. Buffers accept hydrogen ions when a solution shifts toward acidic and drop hydrogen ions when a solution shifts toward basic. Most buffers are a combination of a weak acid and a weak base, which combine in a reversible fashion.

One buffer that is important to the maintenance of blood pH is called carbonic acid (H2CO3). Carbonic acid forms when carbon dioxide reacts with water in the bloodstream. Carbonic acid is an H+ donor, and as such, dissociates into bicarbonate ions (HCO3) and hydrogen ions (H+). This change from carbonic acid to bicarbonate is reversible and continuously goes back and forth, increasing or decreasing the pH of the blood. The other buffer systems in the body also work in the same manner, having both acidic and basic components or states.