Which gas forms carboxyhaemoglobin?

• A. Carbon Monoxide
• B. Carbon Dioxide
• C. Oxygen
• D. Nitrogen

Answer: (A)

Carbon monoxide forms carboxyhaemoglobin because it binds to the iron in hemoglobin with an affinity far greater than that of oxygen. This strong, preferential binding quickly occupies the binding sites on hemoglobin, drastically reducing the blood’s capacity to carry oxygen. It also shifts the oxygen dissociation curve to the left, so the remaining oxygen is held more tightly and released to tissues less readily. The net effect is severe tissue hypoxia even when lung oxygen levels are adequate, which is why CO poisoning is so dangerous.

Carbon dioxide wouldn’t form carboxyhaemoglobin; it forms carbaminohemoglobin by attaching to amino groups on the hemoglobin molecule. Oxygen, on the other hand, binds to the heme iron to form oxyhaemoglobin, enabling oxygen transport. Nitrogen does not form a stable bond with hemoglobin under normal conditions.