Carbon monoxide is formed by the incomplete oxidation of carbon. When inspired, it rapidly diffuses into the bloodstream. Here it has a 240 times greater affinity for haemoglobin than oxygen. Hence, carboxyhaemoglobin is formed preferentially. Carboxyhaemoglobin dissociates less readily than oxyhaemoglobin. Consequently, there is less available oxygen-carrying capacity within the blood. The haemoglobin dissociation curve is shifted to the left. The dissociation reaction can be encouraged by administering 100% oxygen; the half life is reduced from four hours in air to forty minutes.
Carbon monoxide binds avidly to other haem-containing compounds. The intracellular cytochrome electron carrying chain may be affected resulting in 'sick cell syndrome'. Carbon monoxide bound to the chain dissociates up to 24 hours after exposure; this may lead to post-intoxication encephalopathy.
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