The Oxygen Dissociation Curve
The oxygen dissociation curve is a graph that shows the percent saturation of hemoglobin 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 erythrocytes are fifty percent saturated with oxygen.  (Bouverot, 1985).  The purpose of an oxygen dissociation curve is to show the equilibrium of oxyhemoglobin and nonbonded hemoglobin at various partial pressures (Oxygen Dissociation Curve).  At high partial pressures of oxygen, usually in the lungs, hemoglobin binds to oxygen to form oxyhemoglobin.  When the blood is fully saturated all the erythrocytes are in the form of oxyhemoglobin.  As the erythrocytes travel to tissues deprived of oxygen the partial pressure of oxygen will decrease.  Consequently, the oxyhemoglobin releases the oxygen to form hemoglobin (Schmidt-Nielsen, 1997).  

The sigmoid shape of the oxygen dissociation curve is a result of the cooperative binding of oxygen to the four polypeptide chains.  Cooperative binding is the characteristic of a hemoglobin to have a greater ability to bind oxygen after a subunit has bound oxygen (Oxygen Dissociation Curve).  Thus, hemoglobin is most attracted to oxygen when three of the four polypeptide chains are bound to oxygen.  

Hypothetical oxygen dissociation graph.  Changes in temperature, pH, and organic phosphates like DPG directly effect the dissociation of oxygen.  Image used with permission of The American Society of Health-System Pharmacists, Inc


Factors that Influence Oxygen Binding

  • Temperature- Increasing the temperature denatures the bond between oxygen and hemoglobin, which increases the amount of oxygen and hemoglobin and decreases the concentration of oxyhemoglobin (Schmidt-Nielsen, 1997).  The dissociation curve shifts to the right.

  • pH- A decrease in pH (increase in acidity) 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 and it is more pronounced in animals of smaller size due to the increase in sensitivity to acid (Schmidt-Nielsen, 1997).  The dissociation curve shifts to the right.

  • Organic Phospates- 2,3-Diphosphoglycerated (DPG) is the primary organic phosphate in mammals.  DPG binds to hemoglobin which rearranges the hemoglobin into the T-state, thus decreasing the affinity of oxygen for hemoglobin (T and R State).  The curve shifts to the right.


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Literature Cited     

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Last Updated November 28, 1999

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