Which of the following has greatest affinity for haemoglobin ?

Options

A. CO
B. NO
C. `O_(2)`
D. `CO_(2)`

Answer

To determine which of the following substances has the greatest affinity for hemoglobin, we need to analyze the options provided: Carbon monoxide, Nitrogen oxide, Oxygen, and Carbon dioxide.

### Step-by-Step Solution:

1. **Understanding Hemoglobin's Role**: Hemoglobin is a protein in red blood cells that carries oxygen from the lungs to the rest of the body and returns carbon dioxide from the body back to the lungs.

2. **Analyzing the Options**:
- **Carbon Monoxide (CO)**: It binds to hemoglobin with a very high affinity, even higher than oxygen, which can lead to carbon monoxide poisoning.
- **Nitrogen Oxide (NO)**: This molecule also binds to hemoglobin, but it has a unique property of binding rapidly and irreversibly, making it a strong contender for the highest affinity.
- **Oxygen (O2)**: This is the primary molecule that hemoglobin is designed to transport. It has a significant affinity for hemoglobin but is less than that of carbon monoxide and nitrogen oxide.
- **Carbon Dioxide (CO2)**: While it does bind to hemoglobin, its affinity is lower compared to the other options listed.

3. **Comparative Affinity**:
- Among the options, Nitrogen oxide (NO) is noted for its rapid and strong binding to hemoglobin, making it the substance with the greatest affinity.
- Carbon monoxide is also a strong binder but is not as rapid or irreversible as nitrogen oxide.

4. **Conclusion**: Based on the analysis, Nitrogen oxide has the greatest affinity for hemoglobin compared to the other options.

### Final Answer:
**Nitrogen oxide (NO)** has the greatest affinity for hemoglobin.

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