Acid-Base Equilibria and the Bicarbonate Buffer System in Blood

Chapter 10: Acids, Bases, and Equilibrium

The Carbonic Acid–Bicarbonate Buffer System in Blood

The carbonic acid–bicarbonate buffer system is a crucial physiological mechanism that helps maintain the pH of blood within a narrow, healthy range (typically 7.35–7.45). This buffer system minimizes changes in blood pH when acids or bases are introduced.

• Buffer System Components: The system consists of carbonic acid (H2CO3) and bicarbonate ion (HCO3−).

• Buffer Action: When excess H+ (acid) is present, bicarbonate ions neutralize it, forming carbonic acid. When excess OH− (base) is present, carbonic acid donates H+ to neutralize it, forming bicarbonate.

Equilibrium Equation for the Buffer System

The buffer system is governed by the following equilibrium equation:

$\mathrm{CO}_2 (g) + \mathrm{H}_2O (l) \rightleftharpoons \mathrm{H}_2\mathrm{CO}_3 (aq) \rightleftharpoons \mathrm{H}^+ (aq) + \mathrm{HCO}_3^- (aq)$

• CO2 and H2O: Carbon dioxide from cellular respiration dissolves in blood and reacts with water to form carbonic acid.

• Carbonic Acid Dissociation: Carbonic acid partially dissociates into hydrogen ions (H+) and bicarbonate ions (HCO3−).

• pH Regulation: The relative concentrations of H+ and HCO3− determine the blood's pH.

Effect of Hyperventilation on pH Balance

Hyperventilation is rapid or deep breathing that causes excessive loss of CO2 from the body. This affects the buffer equilibrium and blood pH:

• CO2 Decrease: Hyperventilation reduces CO2 levels in the blood.

• Equilibrium Shift: According to Le Châtelier's Principle, the equilibrium shifts left to replace lost CO2, consuming H+ ions and decreasing their concentration.

• Resulting pH Change: The decrease in H+ causes the blood pH to rise (alkalosis).

Summary Table: Effects of Hyperventilation on Blood Buffer System

Condition	CO2 Level	Equilibrium Shift	H+ Concentration	Blood pH
Normal Breathing	Normal	No shift	Normal	7.35–7.45
Hyperventilation	Decreases	Left (consumes H+)	Decreases	Increases (alkalosis)

Example: Buffer Response to Acid Addition

• If a strong acid is added to blood, HCO3− neutralizes the excess H+, forming H2CO3, which can then convert to CO2 and be exhaled.

Key Takeaways:

• The carbonic acid–bicarbonate buffer system is essential for maintaining blood pH.

• The equilibrium equation shows how CO2, H2CO3, H+, and HCO3− are interrelated.

• Hyperventilation shifts the equilibrium, reducing H+ and increasing blood pH (alkalosis).