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The presence of COHb shifts the O2 dissociation curve to the left, thus interfering with the unloading of O2.
The oxygen dissociation curve is a graph that shows the percent saturation of haemoglobin at various partial pressures of oxygen. Commonly a curve may be expressed with the P50 value. This is a value which tells the pressure at which the red blood cells are fifty percent saturated with oxygen. The purpose of an oxygen dissociation curve is to show the equilibrium of oxyhaemoglobin and nonbonded haemoglobin at various partial pressures. At high partial pressures of oxygen, haemoglobin binds to oxygen to form oxyhaemoglobin. When the blood is fully saturated all the red blood cells are in the form of oxyhaemoglobin. As the red blood cells travel to tissues deprived of oxygen the partial pressure of oxygen will decrease. Consequently, the oxyhaemoglobin releases the oxygen to form haemoglobin.
The sigmoid shape of the oxygen dissociation curve is a result of the co-operative binding of oxygen to the four polypeptide chains. Co-operative binding is the characteristic of a haemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen. Thus, haemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.
If the haemoglobin level is halved, the oxygen content of arterial blood will be halved. Carbon monoxide (CO) interferes with the O2 transport function of blood by combining with Hb to form carboxyhaemoglobin (COHb). CO has about 240 times the affinity of O2 for Hb. For this reason, small amounts of CO can tie up a large proportion of the Hb in the blood, thus making it unavailable for O2 carriage. If this happens, the Hb concentration and PO2 of blood may be normal, but its O2 concentration is grossly reduced.
The presence of COHb also shifts the O2 dissociation curve to the left, thus interfering with the unloading of O2.
This is an additional feature of the toxicity of CO.