Question

What is the compound formed when $CO$ combines with hemoglobin?

Answer 1

Haemoglobin is a protein whose main function is to transport oxygen from the lungs to the tissues and to transport carbon dioxide gas from the tissues to the lungs. Carbon monoxide is a colorless, tasteless, odorless and flammable gas. It is harmful when inhaled because it displaces oxygen in the blood.

Complete answer:
Carbon monoxide combines with hemoglobin to form carboxyhemoglobin. It also acts to increase the stability of the bond between oxygen and hemoglobin, reducing the ability of the hemoglobin molecule to release oxygen bound to other oxygen binding sites. The overall result is a decrease in the oxygen available to the tissues present in the body. Carboxyhemoglobin is a stable complex of carbon monoxide and hemoglobin that forms in red blood cells on contact with carbon monoxide. Compared to oxygen, carbon monoxide binds with approximately $240$ times higher affinity. Increase in the levels of carboxyhemoglobin results in various degrees of asphyxiation, including death. It does not dissociate readily. As a result, the oxygen carrying power of blood gets limited. A high level of carboxyhemoglobin is the cause of death in carbon monoxide poisoning as from car exhaust.
Therefore, the compound formed when $CO$ combines with hemoglobin is carboxyhemoglobin.

Note:
A carbon monoxide blood test helps to detect carbon monoxide poisoning. Poisoning can happen if we breathe air that contains more carbon monoxide. The test measures the amount of hemoglobin that has bonded with carbon monoxide gas. This is known as the carboxyhemoglobin level.