30.15 : Bicarbonate-Carbonic Acid Buffer

The carbonic acid-bicarbonate buffer system is critical for maintaining the body's pH balance. It operates on the equilibrium:

H₂CO₃ ⇋ H⁺ + HCO₃^-

In this system, bicarbonate ions (HCO₃⁻) act as weak bases, and carbonic acid (H₂CO₃) serves as a weak acid. This dynamic equilibrium enables the system to respond effectively to changes in pH.

When hydrogen ion (H⁺) levels increase, causing a drop in pH, the equilibrium shifts to the left, converting hydrogen ions into carbonic acid. The enzyme carbonic anhydrase rapidly converts carbonic acid into carbon dioxide (CO₂) and water, allowing CO₂to be exhaled via the lungs. This mechanism helps lower H⁺levels and restore pH.

Conversely, when hydrogen ion levels decrease, resulting in a rise in pH, the equilibrium shifts to the right. Carbonic acid dissociates into H⁺and HCO₃⁻, replenishing hydrogen ions and stabilizing pH. This balance ensures the body maintains a steady pH, which is vital for normal physiological function.

The effectiveness of the carbonic acid-bicarbonate buffer system depends on the availability of its buffering components. If a large acid load depletes bicarbonate ions (the "alkaline reserve"), the system's buffering capacity diminishes, leading to significant pH changes.

Although the body typically has ample bicarbonate reserves, the system's efficacy can be compromised in respiratory or metabolic dysfunction cases, where either CO₂ excretion or bicarbonate regulation is impaired.

The carbonic acid-bicarbonate buffer system plays a central role in maintaining acid-base homeostasis. By dynamically responding to changes in hydrogen ion concentrations, this system helps preserve the stability necessary for vital physiological processes.