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 regarding which gas combines with hemoglobin to form a very stable compound and reduces the oxygen carrying capacity of blood, we can follow these steps:

### Step 1: Identify the Gases
The question provides four gases:
1. Carbon Dioxide (CO₂)
2. Carbon Monoxide (CO)
3. Sulfur Dioxide (SO₂)
4. Nitrogen Dioxide (NO₂)

### Step 2: 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. It can also bind with other gases.

### Step 3: Analyze Each Gas
- **Carbon Dioxide (CO₂)**: It is transported in the blood but does not form a stable compound with hemoglobin that significantly reduces oxygen carrying capacity.
- **Carbon Monoxide (CO)**: This gas binds with hemoglobin to form carboxyhemoglobin, which is a very stable compound. This binding significantly reduces hemoglobin's ability to carry oxygen, leading to suffocation.
- **Sulfur Dioxide (SO₂)**: While it can affect the respiratory system, it does not form a stable compound with hemoglobin.
- **Nitrogen Dioxide (NO₂)**: Similar to sulfur dioxide, it does not form a stable compound with hemoglobin.

### Step 4: Conclusion
Based on the analysis, carbon monoxide (CO) is the gas that combines with hemoglobin to form a very stable compound (carboxyhemoglobin) and reduces the oxygen carrying capacity of blood.

### Final Answer
The correct answer is **Carbon Monoxide (CO)**.

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