Carboxyhemoglobin

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Carboxyhemoglobin (COHb) is a stable complex of carbon monoxide and hemoglobin that forms in red blood cells when carbon monoxide is inhaled, and hinders delivery of oxygen to the body. Tobacco smoking (through carbon monoxide inhalation) raises the blood levels of COHb.

In large quantities, the effect of COHb is death - known medically as carboxyhemoglobinemia or carbon monoxide poisoning. However in smaller quantities COHb leads to oxygen deprivation of the body causing tiredness, dizziness and unconsciousness.

Hemoglobin bonds to carbon monoxide preferentially (200:1 more so) compared to bonding to oxygen, so effectively, COHb will not release the carbon monoxide, and therefore hemoglobin will not be available to transport oxygen from the lungs to the rest of the body. However, animals, such as a human, should survive with very small amounts of COHb in their blood with very little or no observable effects.

COHb has a half-life in the blood of 4 to 6 hours. COHb increases risk of blood clot. It is thought that through this mechanism smoking increases the risk of having an ischemic stroke.