Which of the following has greatest affinity for haemoglobin ?

Options

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

Answer

To determine which substance has the greatest affinity for hemoglobin, we need to analyze the binding characteristics of hemoglobin with various gases, particularly oxygen (O2) and carbon monoxide (CO).

### Step-by-Step Solution:

1. **Understanding Hemoglobin's Function**: Hemoglobin is a protein in red blood cells that is responsible for transporting oxygen from the lungs to the rest of the body and returning carbon dioxide (CO2) from the body back to the lungs.

2. **Binding Affinity**: The affinity of hemoglobin for different gases can be compared. Hemoglobin binds to oxygen to form oxyhemoglobin (HbO2), which is essential for oxygen transport.

3. **Comparing Gases**:
- Oxygen (O2) binds to hemoglobin, but there are other gases that can also bind to hemoglobin.
- Carbon monoxide (CO) is a significant competitor for binding sites on hemoglobin.

4. **Affinity of Carbon Monoxide**:
- Carbon monoxide has a much higher affinity for hemoglobin compared to oxygen. Specifically, it binds to hemoglobin to form carboxyhemoglobin (HbCO) with an affinity that is approximately 200 times greater than that of oxygen.
- This means that when both gases are present, CO will preferentially bind to hemoglobin, preventing oxygen from binding and leading to serious health issues such as suffocation.

5. **Conclusion**: Based on the binding affinities, carbon monoxide (CO) has the greatest affinity for hemoglobin.

### Final Answer:
**Carbon monoxide (CO) has the greatest affinity for hemoglobin.**