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.
Factors that Influence Oxygen Binding
Increasing the temperature denatures the bond between oxygen and haemoglobin, which increases the amount of oxygen and haemoglobin and decreases the concentration of oxyhemoglobin. The dissociation curve shifts to the right.
A decrease in pH by addition of carbon dioxide or other acids causes a Bohr Shift. A Bohr shift is characterized by causing more oxygen to be given up as oxygen pressure increases. The dissociation curve shifts to the right.
2,3-Diphosphoglycerate (DPG) is the main primary organic phosphate. DPG binds to haemoglobin which rearranges the haemoglobin into the T-state, thus decreasing the affinity of oxygen for haemoglobin (T and R State). The curve shifts to the right.