Which one among the following gases readily combines with the haemoglobin of the blood ?

Carbon monoxide (CO) is a colorless, odorless gas that readily combines with the hemoglobin in human blood. While hemoglobin naturally has a high affinity for oxygen to facilitate transport to tissues [3], its affinity for carbon monoxide is significantly higher—approximately 200 to 270 times greater than its affinity for oxygen. When inhaled, CO competitively binds to the heme group of hemoglobin to form a stable complex called carboxyhemoglobin (COHb) [2]. This binding effectively displaces oxygen and inhibits the blood's ability to transport oxygen to vital organs like the heart and brain, leading to hypoxic stress or asphyxiation [2]. Although other gases like sulfur dioxide can interact with hemoglobin under specific conditions to form sulfhemoglobin, carbon monoxide is the primary gas known for its rapid and high-affinity binding that outcompetes oxygen in the respiratory system [5].

Sources

1. [3] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/carbon-monoxide
2. [1] https://www.cdc.gov/niosh/chemicals/pel88/pell-pages/630-08.html
3. [2] Science , class X (NCERT 2025 ed.) > Chapter 5: Life Processes > Do You Know? > p. 90
4. [5] https://www.branford-ct.gov/FAQ.aspx?QID=69