In order to keep blood pH at 7.4, the HCO3: H2CO3 ratio should be:
## **Core Concept**
The question tests understanding of acid-base balance in the body, specifically the bicarbonate buffering system. This system is crucial for maintaining blood pH and involves the ratio of bicarbonate (HCO3-) to carbonic acid (H2CO3). The Henderson-Hasselbalch equation is key to understanding this concept: pH = pKa + log([A-]/[HA]), where [A-] is the concentration of the conjugate base (HCO3-) and [HA] is the concentration of the weak acid (H2CO3).
## **Why the Correct Answer is Right**
The Henderson-Hasselbalch equation can be applied to the bicarbonate buffering system. The pKa of carbonic acid is approximately 6.1. To maintain a blood pH of 7.4, the equation becomes 7.4 = 6.1 + log([HCO3-]/[H2CO3]). Solving for log([HCO3-]/[H2CO3]) gives 1.3. Therefore, [HCO3-]/[H2CO3] = 10^1.3, which is approximately 20:1. This ratio ensures that the buffering system can effectively maintain blood pH at 7.4.
## **Why Each Wrong Option is Incorrect**
- **Option A:** A ratio of 1:20 would imply a pH lower than 6.1, which is acidic and far from the normal blood pH of 7.4.
- **Option B:** A ratio of 1:1 would not be sufficient to achieve a pH of 7.4 given the pKa of carbonic acid.
- **Option D:** A ratio of 40:1 would result in a higher pH than 7.4, as it would imply a more alkaline condition.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the bicarbonate buffering system is one of the body's primary mechanisms for maintaining acid-base balance. The normal ratio of HCO3- to H2CO3 is approximately 20:1, and any significant deviation from this ratio can lead to acidosis or alkalosis. Understanding this ratio and the Henderson-Hasselbalch equation is crucial for assessing and managing acid-base disorders.
## **Correct Answer:** . 20:1.