How does bicarbonate act as a buffer in the blood when [H+] increases?

Bicarbonate serves as a crucial buffer in the blood, and its role becomes particularly important when the concentration of hydrogen ions ([H+]) increases, which can occur during metabolic acidosis or other conditions that lead to acid accumulation. When [H+] rises, bicarbonate ions in the blood interact with these protons (H+) to form carbonic acid (H2CO3). This reaction is catalyzed by the enzyme carbonic anhydrase and can be represented by the equilibrium reaction:

HCO3^- + H^+ ⇌ H2CO3

The formation of carbonic acid helps to moderate changes in pH by reducing the free hydrogen ion concentration, effectively neutralizing some of the excess protons. Carbonic acid can then dissociate back into bicarbonate and hydrogen ions, which allows the system to respond dynamically to changes in pH levels.

By acting in this manner, bicarbonate provides a buffer system that helps maintain the blood pH within a narrow range necessary for proper physiological function. This buffering mechanism is vital for homeostasis, particularly in maintaining the acid-base balance in the body.