Buffer systems in the human body are composed of a weak acid or base, along with its conjugate acid or base. This system prevents large fluctuations in the body and cells' pH due to an addition of a strong acid or base.
In the human body, cell mechanisms only operate within a specific range of temperatures and pH levels. Proteins denature and are degraded outside these specific ranges, and enzymes cease to operate, potentially resulting in death.
A buffer system is a collection of molecules that may reversibly bind hydrogen ions. The human blood supply provides a good example. The bicarbonate buffer system maintains a pH of 7.38-7.42 in the arterial blood. In this system, carbonic acid is constantly converting back and forth from bicarbonate, absorbing or releasing hydrogen ions as necessary. Another well-known extracellular buffer is ammonia. Several intracellular buffer systems, such as phosphate or proteins, are also used to absorb excess hydrogen ions within cells.
These buffering mechanisms essentially are the practical application of Le Chatelier's principle, which describes the conversions from conjugate acid to base and vice versa. For example, if blood pH would drop, the body would make up for it by speeding up breathing to release carbon dioxide, which would allow the pH to return to normal.