Which gas has a stronger affinity for hemoglobin than oxygen?

• A. Carbon dioxide
• B. Carbon monoxide
• C. Nitrogen
• D. Sulfur dioxide

Answer: (B)

Carbon monoxide has a much stronger affinity for hemoglobin than oxygen because it binds to the iron in the heme group to form carboxyhemoglobin, effectively taking up the sites where oxygen would bind. This blocks oxygen from binding and also hampers its release to tissues, leading to tissue hypoxia even at low exposure levels. In humans, CO’s affinity for hemoglobin is about 200–250 times greater than that of oxygen, which is why even small amounts of CO can be dangerous and rapid poisoning can occur.

Carbon dioxide does bind to hemoglobin as well, but its affinity is far weaker than oxygen, and most CO2 is carried in the blood as bicarbonate rather than bound to hemoglobin. Nitrogen is largely inert in blood and does not bind to hemoglobin in meaningful amounts. Sulfur dioxide mainly causes respiratory irritation and other chemical effects rather than forming a high-affinity, oxygen-displacing bond with hemoglobin.