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Carboxyhemoglobin (British English: Carboxyhaemoglobin) (COHb) is a stable complex of carbon monoxide and hemoglobin that forms in red blood cells when carbon monoxide is inhaled or produced in normal metabolism. Large quantities of it hinder delivery of oxygen to the body. Tobacco smoking (through carbon monoxide inhalation) raises the blood levels of COHb by a factor of several times from its normal concentrations.
Carbon monoxide as poison
Carbon monoxide binds to haemoglobin preferentially compared to oxygen (approx 240:1), so effectively, COHb will not release the carbon monoxide, and therefore haemoglobin will not be available to transport oxygen from the lungs to the rest of the body.
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.
COHb has a half-life in the blood of 4 to 6 hours, but in cases of poisoning, this can be reduced to 70 to 35 minutes with administration of pure oxygen (the lower number applying when oxygen is administered with 4 to 5% CO2 to cause hyperventilation). In addition, treatment in a Hyperbaric Chamber for CO poisoning is a more effective manner of reducing the half-life of COHb than administering oxygen alone. This treatment involves pressurizing the chamber with pure oxygen at an absolute pressure close to three atmospheres allowing the body's fluids to absorb oxygen and to pass free oxygen on to hypoxic tissues instead of the crippled hemoglobin bonded to CO. In effect, bypassing the need for hemoglobin in the blood.
COHb increases risk of blood clot. It is thought that through this mechanism, smoking increases the risk of having thromboembolic disease.
Pregnant smokers may give birth to babies of a lower birth mass. In addition to vasoconstriction of the placenta, another theory is that since fetal hemoglobin takes up carbon monoxide more readily than in an adult, therefore the fetus of a smoker will suffer from mild hypoxia, potentially retarding its development.
Carbon monoxide as normal biological metabolite
In biology, carbon monoxide is naturally produced by the action of heme oxygenase 1 and 2 on the heme from hemoglobin breakdown. This process produces a certain amount of carboxyhemoglobin in normal persons, even if they do not breathe any carbon monoxide.
Following the first report that carbon monoxide is a normal neurotransmitter in 1993, as well as one of three gasses which naturally modulate inflammatory responses in the body (the other two being nitric oxide and hydrogen sulfide), carbon monoxide has received a great deal of clinical attention as a biological regulator. In many tissues, all three gases are known to act as anti-inflammatories, vasodilators and encouragers of neovascular growth. However, the issues are complex, as neovascular growth is not always beneficial, since it plays a role in tumor growth, and also the damage from wet macular degeneration, a disease for which smoking (a major source of carbon monoxide in the blood, several times more than natural production) increases the risk from 4 to 6 times.
Carbon monoxide as beneficially active substance, and potential pharmaceutical
Studies involving carbon monoxide have been conducted in many laboratories throughout the world for its anti-inflammatory and cytoprotective properties. These properties could potentially be used to prevent the development of a series of pathological conditions including ischemia reperfusion injury, transplant rejection, atherosclerosis, severe sepsis, severe malaria or autoimmunity. Clinical tests involving humans have been performed, however the results have not yet been released.
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