MCQ Detailed View

The human body maintains a nearly constant blood pH of about 7.35 to 7.45, which is slightly basic. This narrow range is essential for the proper functioning of enzymes, transport of oxygen, and overall cellular activity. The primary system that maintains this stable pH is the bicarbonate buffer system (H₂CO₃ – HCO₃⁻).

A buffer is a solution that resists significant changes in pH when small amounts of acid or base are added. The bicarbonate buffer system consists of a weak acid, carbonic acid (H₂CO₃), and its conjugate base, bicarbonate ion (HCO₃⁻). The system works through the following equilibrium:

$$\text{H₂CO₃ ⇌ H⁺ + HCO₃⁻}$$

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  When excess acid (H⁺ ions) enters the blood, bicarbonate ions combine with hydrogen ions to form carbonic acid, reducing the rise in acidity.

  
  


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  When excess base (OH⁻ ions) enters the blood, carbonic acid reacts with hydroxide ions to form water and bicarbonate, preventing a sharp increase in pH.

  
  

This dynamic equilibrium provides the body with a rapid mechanism to stabilize pH changes. The bicarbonate system is also closely linked with the respiratory system, since carbon dioxide (CO₂) dissolves in blood to form carbonic acid:

$$\text{CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻}$$

The lungs regulate the level of CO₂ by adjusting breathing rate, while the kidneys control the concentration of bicarbonate ions by excretion or reabsorption. Together, these processes provide powerful control over blood pH.

Other buffer systems, such as phosphate buffer (H₂PO₄⁻/HPO₄²⁻) and protein buffers (hemoglobin, plasma proteins), also contribute, but their effect is smaller compared to the bicarbonate system.

Therefore, the H₂CO₃ – HCO₃⁻ buffer system is the most important buffer in human blood and is essential for maintaining homeostasis.