Explain the bicarbonate buffer system and its relevance to pH regulation.

• A. It uses CO2 and bicarbonate to neutralize acids; shifting balance maintains blood pH
• B. It uses oxygen and carbon dioxide to buffer acids
• C. It buffers pH by binding potassium
• D. It is unrelated to acid-base balance

Answer: (A)

The main idea here is that the bicarbonate buffer system relies on carbon dioxide and bicarbonate to stabilize hydrogen ion levels in the blood, keeping pH within a narrow range. Metabolic CO2 produced by tissues dissolves in blood and, with the help of carbonic anhydrase, forms carbonic acid that quickly dissociates into bicarbonate and a free hydrogen ion. This creates a flexible pair: bicarbonate (a base) and carbonic acid (which reflects CO2). When an acid adds hydrogen ions, those extra H+ are buffered by bicarbonate to form carbonic acid, which becomes CO2 and water that you can exhale. When a base is added, carbonic acid can donate H+ to form bicarbonate, dampening the pH rise. The effectiveness of this system hinges on the ratio of bicarbonate to dissolved CO2 (reflected as carbonic acid): pH rises or falls as this ratio shifts. The lungs rapidly influence this balance by adjusting CO2 levels through ventilation, while the kidneys regulate the amount of bicarbonate (and hydrogen ions) excreted or reabsorbed, providing longer-term control. Together, these adjustments keep blood pH around the normal range.

Oxygen isn’t part of this buffering reaction, and potassium binding isn’t how buffering works, so those options don’t describe the mechanism. The bicarbonate system is indeed central to acid–base balance.