Bicarbonate Buffering System

The bicarbonate buffering system is an important buffer system in the acid-base homeostasis of living things, including humans. As a buffer, it tends to maintain a relatively constant plasma pH and counteract any force that would alter it.

In this system, carbon dioxide (CO₂) combines with water to form carbonic acid (H₂CO₃), which in turn rapidly dissociates to form hydrogen ions and bicarbonate (HCO₃- ) as shown in the reactions below. The carbon dioxide - carbonic acid equilibrium is catalyzed by the enzyme carbonic anhydrase; the carbonic acid - bicarbonate equilibrium is simple proton dissociation/association and needs no catalyst.

Any disturbance of the system will be compensated by a shift in the chemical equilibrium according to Le Chatelier's principle. For example, if one attempted to acidify the blood by dumping in an excess of hydrogen ions (acidemia), some of those hydrogen ions will associate with bicarbonate, forming carbonic acid, resulting in a smaller net increase of acidity than otherwise. This buffering system becomes an even more powerful regulator of acidity when it is coupled with the body's capacity for respiratory compensation, in which breathing is altered to modify the amount of CO₂ in circulation. In the above example, increased ventilation would increase the loss of CO₂ to the atmosphere, driving the equilibria above to the left. The process could continue until the excess acid is all exhaled.

This process is extremely important in the physiology of blood-having animals. It manages the many acid and base imbalances that can be produced by both normal and abnormal physiology. It also affects the handling of carbon dioxide, the constantly produced waste product of cellular respiration.