Is carbonic acid primarily an intracellular or extracellular buffer?

Carbonic acid predominantly functions as an extracellular buffer. This is largely due to its role in maintaining acid-base balance in the blood and extracellular fluid. The carbonic acid-bicarbonate buffer system is vital in regulating pH levels, especially in maintaining homeostasis in the body's fluids.

In the bloodstream, carbonic acid (H2CO3) forms in equilibrium with carbon dioxide (CO2) and bicarbonate (HCO3-). When there's an increase in hydrogen ion concentration in the blood (which could result from metabolic processes), carbonic acid can dissociate to release bicarbonate ions, helping to neutralize the excess acidity. Conversely, if the blood becomes too basic, bicarbonate can react with hydrogen ions to form carbonic acid, helping to lower the pH back to a normal range.

Intracellular buffers primarily consist of proteins and phosphate groups, which play significant roles in pH regulation within cells, but it is the carbonic acid system that is most active in extracellular spaces. This is why the answer identifies carbonic acid as primarily an extracellular buffer.