The most important buffer in the blood consist of :

Options

A. `HCl and Cl^(-)`
B. `H_(2)CO_(3) and H_(2)CO_(3)^(-)`
C. `H_(2)CO_(3) and Cl^(-)`
D. `HCl and HCO_(3)^(-)`

Explanation

To solve the question regarding the most important buffer in the blood, we need to analyze the options provided and understand the role of different buffer systems in the body.

### Step-by-Step Solution:

1. **Understanding Buffers**:
- A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its conjugate base.

2. **Identifying the Options**:
- The options given are:
1. HCl and Cl⁻
2. H₂CO₃ and HCO₃⁻
3. H₂CO₃ and Cl⁻
4. HCl and HCO₃⁻

3. **Analyzing Each Option**:
- **Option 1 (HCl and Cl⁻)**: HCl is a strong acid and does not function as a buffer.
- **Option 2 (H₂CO₃ and HCO₃⁻)**: H₂CO₃ (carbonic acid) is a weak acid, and HCO₃⁻ (bicarbonate ion) is its conjugate base. This combination is known to effectively maintain pH balance in the blood.
- **Option 3 (H₂CO₃ and Cl⁻)**: Cl⁻ is not a conjugate base of a weak acid and does not contribute to buffering capacity.
- **Option 4 (HCl and HCO₃⁻)**: HCl is a strong acid and cannot be part of a buffering system.

4. **Conclusion**:
- The most important buffer system in the blood is the carbonic acid (H₂CO₃) and bicarbonate ion (HCO₃⁻) system. This system plays a crucial role in maintaining the acid-base balance in the body, especially during processes like cellular respiration.

5. **Final Answer**:
- The correct option is **Option 2: H₂CO₃ and HCO₃⁻**.