13. Which one of the following is the most powerful buffer system of blood?
(1) Bicarbonate                                              (2) Phosphate
(3) Proteins                                                    (4) Haemoglobin

Introduction

Maintaining a stable blood pH (~7.35-7.45) is critical for proper cell function and overall survival. The human body employs several buffer systems to resist changes in pH caused by acids or bases introduced or produced metabolically. Among these, the bicarbonate buffer system is recognized as the most powerful and primary buffer system of blood. This article explains why bicarbonate dominates in blood pH regulation compared to the phosphate system, proteins, and hemoglobin, outlining its mechanism of action and physiological importance.

Overview of Buffer Systems in Blood

A blood buffer system consists of a weak acid and its conjugate base that can neutralize excess hydrogen ions (H+) or hydroxide ions (OH−), minimizing drastic pH changes. The major buffer systems in blood include:

• Bicarbonate buffer system (H2CO3 / HCO3−)

• Phosphate buffer system (H2PO4− / HPO42−)

• Protein buffer system (mostly albumin and plasma globulins)

• Hemoglobin buffer system (mainly inside red blood cells)

Why Bicarbonate Is the Most Powerful Buffer

The bicarbonate buffer system consists of a dynamic equilibrium between carbonic acid (H2CO3) and bicarbonate ion (HCO3−):

H++HCO3−↔H2CO3↔CO2+H2OH++HCO3−↔H2CO3↔CO2+H2O

• Dynamic respiratory control: The system is uniquely linked to carbon dioxide removal via the lungs. When excess acid is present, bicarbonate binds H+ to form carbonic acid, which converts to CO2 and water. CO2 is exhaled by the lungs, effectively removing acid from the body.

• Large reservoir: Blood contains a high bicarbonate concentration and can buffer significant acid/base loads.

• Rapid response: Chemical buffering is instantaneous, and respiratory compensation adjusts CO2 levels in minutes, allowing fast control of blood pH.

Roles of Other Buffer Systems

Phosphate Buffer System

• Works mainly in intracellular fluid and kidneys to regulate urinary pH.

• Less abundant and effective in plasma due to lower phosphate concentration.

Protein Buffer System

• Plasma proteins (e.g., albumin) can bind H+ ions.

• Inside red blood cells, hemoglobin also acts as a buffer by binding free H+ released during CO2 transport.

Hemoglobin Buffer System

• Complementary to bicarbonate system, especially within RBCs.

• Helps transport CO2 and maintain acid-base balance locally.

Clinical Significance of Bicarbonate Buffer System

• Acid-Base Disorders: Metabolic acidosis or alkalosis often involve disturbances in bicarbonate levels or respiratory CO2 elimination.

• Kidney and lung function: The kidneys regulate bicarbonate retention and excretion while lungs modulate CO2 exhalation.

• Buffering capacity: The system buffers blood pH efficiently despite daily acid production from metabolism.

Summary Table

Correct answer: (1) Bicarbonate.