The Oxygen Dissociation Curve

• The oxygen dissociation curve shows the rate at which oxygen associates and dissociates, with haemoglobin at different partial pressures of oxygen (pO2).
• Partial pressure of oxygen refers to the pressure exerted by oxygen within a mixture of gases; it is a measure of oxygen concentration.
• Haemoglobin is referred to as being saturated when all of its oxygen binding sites are taken up with oxygen (when it contains four oxygen molecules).
• The ease with which haemoglobin binds and dissociates with oxygen can be described as its affinity for oxygen. Therefore haemoglobin's affinity for oxygen changes at different partial pressures of oxygen:

  • When haemoglobin has a high affinity it binds easily and dissociates slowly
  • When haemoglobin has a low affinity for oxygen it binds slowly and dissociates easily
• In water, we would expect oxygen to dissolve at a constant rate, providing a straight line on a graph. With haemoglobin, oxygen binds at different rates as the pO2 changes; hence the resulting curve.
• The shape of haemoglobin makes it difficult for the first oxygen molecule to bind. This means that binding of the first oxygen occurs slowly, which explains the shallow curve at the bottom of the graph.
• Binding of the first oxygen molecule induces a conformational change in the haemoglobin protein. This makes it easier for other oxygen molecules to bind, which speeds up binding. This is known as cooperative binding, and it explains the steep section in the middle of the graph.
• As the haemoglobin molecule approaches saturation it takes longer for the fourth oxygen molecule to bind due to the shortage of remaining binding sites. This explains why the curve levels off at the top.
• When the curve is read from left to right, it provides information about the rate at which haemoglobin binds to oxygen at different partial pressures of oxygen.
• Low pO2 (in oxygen depleted tissues):

  • Haemoglobin has a low affinity for oxygen at low pO2 , therefore oxygen binds slowly.
  • Haemoglobin cannot pick up oxygen as blood passes through oxygen-depleted tissues. The saturation percentage is low.
• Medium pO2 (in respiring tissues):

  • Oxygen binds more easily to haemoglobin
  • Saturation increases quickly, and a small increase in pO2 causes a large increase in haemoglobin saturation
• High pO2 (in lungs):

  • Haemoglobin has a high affinity for oxygen at high pO2, therefore oxygen binds rapidly
  • Haemoglobin can pick up oxygen and become saturated as blood passes through the lungs.  The saturation percentage is high
  • Note that at this point on the graph increasing the pO2 by a large amount only has a small effect on the percentage saturation of haemoglobin because most oxygen binding sites on haemoglobin are already occupied
• When the curve is read from left to right, it provides information about the rate at which haemoglobin dissociates from oxygen at different partial pressures of oxygen.
• High pO2 (in lungs):

  In the lungs, where pO2 is high, there is very little dissociation of oxygen from haemoglobin. The saturation percentage is high
• Medium pO2 (respiring tissues):

  • At medium pO2, oxygen dissociates readily from haemoglobin, which is important for cellular respiration
  • A small decrease in pO2 causes a large decrease in percentage saturation of haemoglobin. This leads to the release of plenty of oxygen to the cells.
• Low pO2 (in oxygen depleted tissues):

  • At low pO2 dissociation slows again.
  • There are few oxygen molecules left on the binding sites, and the release of the final oxygen molecule becomes more difficult (in a similar way to the slow binding of the first oxygen molecule).
• Changes in the oxygen dissociation curve as a result of carbon dioxide levels are known as the Bohr effect, or Bohr shift.
• When the partial pressure of carbon dioxide in the blood is high, the affinity of haemoglobin for oxygen is reduced. This happens in respiring tissues, where cells produce CO2 as a waste product.
• In red blood cells, CO2 combines with water to form carbonic acid, which dissociates into hydrogen carbonate ions and hydrogen ions. The hydrogen ions bind to haemoglobin, causing the release of oxygen. This is advantageous because it means that haemoglobin gives up its oxygen more readily in respiring tissues where it is needed. On a graph showing the dissociation curve, the curve shifts to the right when CO2 levels increase. Therefore at any given partial pressure of oxygen, the percentage saturation of haemoglobin is lower at higher levels of CO2.