Question

The bicarbonate buffer is a principal buffering system in blood plasma. The ratio of the bicarbonate to carbonic acid required to maintain pH of plasma at 7.4 is:

• A. 1:2
• B. 2:1
• C. 20:1
• D. 1:20

Hint:

The large excess of bicarbonate over carbonic acid (20:1 ratio) provides a high capacity to buffer acidic metabolic wastes (such as lactic acid), which are more commonly produced in metabolic processes than alkaline ones.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The pH of blood plasma is maintained within a narrow physiological range of 7.35 to 7.45, primarily regulated by the carbonic acid-bicarbonate buffering system.
Key Formula or Approach:
The pH of a buffer system is calculated using the Henderson-Hasselbalch equation:
\[ \text{pH} = \text{pK}_a + \log_{10} \left( \frac{[\text{Base}]}{[\text{Acid}]} \right) \]
For blood plasma, the conjugate base is bicarbonate (\(\text{HCO}_3^-\)) and the weak acid is dissolved carbon dioxide/carbonic acid (\(\text{H}_2\text{CO}_3\)).
The apparent \(\text{pK}_a\) for this system in blood is approximately 6.1.

Step 2: Detailed Explanation:
We substitute the physiological pH of 7.4 and the \(\text{pK}_a\) of 6.1 into the equation:
\[ 7.4 = 6.1 + \log_{10} \left( \frac{[\text{HCO}_3^-]}{[\text{H}_2\text{CO}_3]} \right) \]
Subtracting 6.1 from both sides gives:
\[ 1.3 = \log_{10} \left( \frac{[\text{HCO}_3^-]}{[\text{H}_2\text{CO}_3]} \right) \]
Converting from logarithmic to exponential form:
\[ \frac{[\text{HCO}_3^-]}{[\text{H}_2\text{CO}_3]} = 10^{1.3} \approx 19.95 \approx 20 \]
Thus, the concentration of bicarbonate ions must be approximately 20 times greater than that of carbonic acid.

Step 3: Final Answer:
The ratio of bicarbonate to carbonic acid required to maintain blood plasma pH at 7.4 is 20:1.