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Why can carbon monoxide poison tissues even when oxygen is present in the blood?

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Carbon monoxide poisons tissues even when oxygen is present in the blood because of its chemical interaction with hemoglobin, the protein responsible for transporting oxygen throughout the body.12

Hemoglobin has a significantly greater affinity for carbon monoxide than it does for oxygen. When carbon monoxide is inhaled, it binds to the iron atoms within the hemoglobin molecule to form a highly stable compound called carboxyhemoglobin. Because this bond is so stable, it effectively occupies the sites where oxygen would normally attach, preventing the hemoglobin from binding to and transporting oxygen to the body's tissues.321

Consequently, even if oxygen is present in the blood, the available hemoglobin is "blocked" by carbon monoxide, leading to a state of internal asphyxiation where essential oxygen cannot reach the organs and tissues that require it to function.142