The pH of blood 7.26 to 7.42 is maintained by buffer:

Question

Accepted Answer

The pH of blood is tightly regulated between 7.26 and 7.42, and this is primarily maintained by the bicarbonate buffer system, which is represented by Option 1: $H_{2}CO_{3}/HCO_{3}^{-}$.

### Explanation for Option 1:

1. **Bicarbonate Buffer System**: The bicarbonate buffer system is the most significant buffer system in the blood. It consists of carbonic acid ($H_{2}CO_{3}$) and bicarbonate ions ($HCO_{3}^{-}$). This system is crucial because it can quickly respond to changes in pH by shifting the equilibrium between carbonic acid and bicarbonate ions.

2. **Mechanism**: When the blood becomes too acidic (low pH), bicarbonate ions can neutralize excess hydrogen ions, forming carbonic acid, which can then be converted to carbon dioxide and water, with the carbon dioxide being exhaled. Conversely, if the blood becomes too basic (high pH), carbonic acid can dissociate to release hydrogen ions, thus lowering the pH.

3. **Physiological Relevance**: The bicarbonate buffer system is directly linked to the respiratory system and the kidneys, which help regulate the levels of carbon dioxide and bicarbonate in the blood, respectively. This makes it highly effective in maintaining the pH within the narrow range necessary for proper physiological function.

### Explanation for Why Other Options are Incorrect:

2. **Option 2: $H_{3}PO_{4}/H_{2}PO_{4}^{-}$**: The phosphate buffer system is more relevant in the intracellular environment and in the kidneys rather than in the blood. It is not the primary buffer system for maintaining blood pH.

3. **Option 3: $CH_{3}COOH/CH_{3}COO^{-}$**: The acetic acid/acetate buffer system is not significant in human physiology for blood pH regulation. It is more commonly used in laboratory settings for buffer solutions.

4. **Option 4: $NH_{2}CH_{2}COOH/NH_{2}CH_{2}COO^{-}$**: This represents an amino acid buffer system, which is more relevant in protein buffering within cells and not a primary buffer system for blood pH.

In conclusion, the bicarbonate buffer system ($H_{2}CO_{3}/HCO_{3}^{-}$) is the primary and most effective buffer system for maintaining the pH of blood within the narrow range of 7.26 to 7.42, making Option 1 the correct answer.