Which of the following combines with haemoglobin in blood?
(a) Carbon dioxide
(b) carbon monoxide
(c) oxygen
(d) methane

Options

A. Carbon dioxide
B. carbon monoxide
C. oxygen
D. methane

Answer

To determine which of the following substances combines with hemoglobin in blood, we will analyze each option provided:

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

2. **Analyzing Each Option**:
- **(a) Carbon Dioxide**: Hemoglobin does bind to carbon dioxide, but it does so to a lesser extent compared to oxygen and carbon monoxide. It forms a compound known as carboxyhemoglobin.
- **(b) Carbon Monoxide**: Hemoglobin has a very high affinity for carbon monoxide, even greater than that for oxygen. When hemoglobin binds with carbon monoxide, it forms a stable compound called carboxyhemoglobin, which does not easily dissociate.
- **(c) Oxygen**: Hemoglobin binds with oxygen to form oxyhemoglobin, but its affinity for oxygen is lower than that for carbon monoxide.
- **(d) Methane**: Hemoglobin does not combine with methane at all.

3. **Conclusion**: Based on the affinities of hemoglobin for these substances, the correct answer is (b) carbon monoxide, as it combines most readily with hemoglobin in the blood.

### Final Answer:
(b) carbon monoxide