Oxygen dissociation curve

• Haemoglobins affinity for oxygen is dependent on some conditions:

  • The partial pressure of oxygen (pO2)
  • Haemoglobin saturation
  • The partial pressure of carbon dioxide (pCO2)
• The partial pressure of a gas is a measure of the concentration of that gas in a mixtures of gases or in a liquid
• Partial pressure is based on how much pressure that gas contributes to the overall pressure exerted by the mixture of gases
• Oxygen partial pressure (pO2) - The concentration of oxygen in the cells
• The higher the concentration of oxygen the higher the pO2, and as pO2 increases haemoglobins affinity increases. This means:

  • Haemoglobin will have a higher affinity for O2 in oxygen-rich areas (eg lungs) where there is a high pO2, promoting oxygen loading
• The higher the concentration of oxygen the higher the pO2, and as pO2 increases haemoglobins affinity increases. This means:

  • Haemoglobin will have a lower affinity for O2 in oxygen-starved areas (eg respiring tissues) where there is a low pO2, promoting oxygen unloading
• The effect of po2 on the affinity of oxygen allows oxygen to be transported from the lungs (lots of oxygen) to the respiring tissues (oxygen is limited)

  • Allows oxygen to be transported to the cells where oxygen is needed the most
• The saturation of haemoglobin can also affect its affinity for oxygen

  • This is because as each oxygen molecule binds it changes the shape of the haemoglobin in a way that makes it easier for further oxygen molecules to bind (co-operative binding)
• As the saturation of haemoglobin increases, it becomes harder for the final oxygen molecules to bind

  • This means the saturation of haemoglobin which increasing pO2 is not linear
  • This is shown in the s-shape of oxygen dissociation curves
• Saturation of haemoglobin with oxygen - When haemoglobin is holding the maximum amount of oxygen it can bind
• The shape of the haemoglobin molecule makes it difficult for the first oxygen molecule to bind to one of the sites on its four polypeptide subunits because they are closely united.

  • Therefore at low oxygen concentrations, little oxygen binds to haemoglobin
  • The gradient of the curve is shallow initially
• Once the first oxygen molecule is bound, it changes the quaternary structure of the haemoglobin molecule, causing it to change shape

  • This change makes it easier for the other subunits to bind to an oxygen molecule