Which of the following gases combines with haemoglobin to form a very stable compound and reduces the oxygen carrying capacity of blood?

Options

A. `CO_2`
B. CO
C. `SO_2`
D. `NO_2`

Answer

To solve the question, we need to identify which gas combines with hemoglobin to form a stable compound that reduces the oxygen-carrying capacity of blood.

### Step-by-Step Solution:

1. **Understand Hemoglobin's Function**:
- Hemoglobin is a protein in red blood cells responsible for transporting oxygen from the lungs to the rest of the body.

2. **Identify the Gases**:
- The question refers to gases that can bind to hemoglobin. Common gases that interact with hemoglobin include oxygen (O₂), carbon dioxide (CO₂), and carbon monoxide (CO).

3. **Research the Interaction of Gases with Hemoglobin**:
- Oxygen binds to hemoglobin to form oxyhemoglobin, which is a reversible process.
- Carbon dioxide can bind to hemoglobin, but it does not form a very stable compound that significantly reduces oxygen transport.
- Carbon monoxide, on the other hand, binds to hemoglobin to form carboxyhemoglobin.

4. **Evaluate the Stability of the Compounds**:
- Carboxyhemoglobin (formed from carbon monoxide and hemoglobin) is a very stable compound. The binding of carbon monoxide to hemoglobin is an irreversible process, meaning once it binds, it does not easily release.

5. **Consider the Affinity of the Gases**:
- Carbon monoxide has a much higher affinity for hemoglobin than oxygen—over 200 times greater. This means that even small amounts of carbon monoxide can significantly reduce the blood's ability to carry oxygen.

6. **Conclusion**:
- Based on the information gathered, the gas that combines with hemoglobin to form a very stable compound and reduces the oxygen-carrying capacity of blood is **carbon monoxide**.

### Final Answer:
**Carbon monoxide (CO)**