Which of the following gases makes the most stable combination with the haemoglobin of RBCs?

Options

A. (a) `CO_(2)`
B. (b) `CO`
C. (c) `O_(2)`
D. (d) `N_(2)`

Answer

To determine which gas makes the most stable combination with the hemoglobin of red blood cells (RBCs), we need to analyze the interactions of various gases with hemoglobin. Here’s a step-by-step breakdown of the solution:

### Step 1: Understand Hemoglobin's Role
Hemoglobin is a protein in red blood cells that binds to gases, primarily oxygen, and transports them throughout the body. It can also bind to other gases like carbon dioxide and carbon monoxide.

**Hint:** Recall the primary function of hemoglobin in gas transport.

### Step 2: Analyze Each Gas
- **Carbon Dioxide (CO2):** When carbon dioxide binds to hemoglobin, it forms carbaminohemoglobin. The affinity of CO2 for hemoglobin is influenced by the pH of the blood (Bohr's effect), but it is not the most stable combination.

**Hint:** Consider how pH levels affect gas binding to hemoglobin.

- **Carbon Monoxide (CO):** Carbon monoxide binds to hemoglobin to form carboxyhemoglobin. This combination is extremely stable and has a much higher affinity for hemoglobin than oxygen—about 240 times greater.

**Hint:** Think about the dangers of carbon monoxide poisoning due to its strong binding affinity.

- **Oxygen (O2):** Oxygen binds to hemoglobin to form oxyhemoglobin. While essential for respiration, its affinity is lower compared to carbon monoxide.

**Hint:** Compare the binding affinities of oxygen and carbon monoxide.

- **Nitrogen (N2):** Nitrogen does not significantly bind to hemoglobin under normal physiological conditions, making it irrelevant in this context.

**Hint:** Consider the inert nature of nitrogen in biological systems.

### Step 3: Compare Affinities
When comparing the affinities:
- Carboxyhemoglobin (CO + hemoglobin) has an affinity 240 times greater than oxyhemoglobin (O2 + hemoglobin).
- Therefore, carbon monoxide forms the most stable combination with hemoglobin.

**Hint:** Recall the significance of binding affinity in determining stability.

### Conclusion
Based on the analysis, the gas that makes the most stable combination with hemoglobin of RBCs is **carbon monoxide (CO)**.

### Final Answer
**B. Carbon monoxide**