Oxygen Dissociation Curve

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•  Oxygen dissociation curves show the affinity of haemoglobin for oxygen

Oxygen is transported throughout the body in red blood cells, which contain an oxygen-binding protein called haemoglobin

  • Haemoglobin is composed of four polypeptide chains, each with an iron-containing heme group that reversibly binds oxygen
  • As such, each haemoglobin can reversibly bind up to four oxygen molecules (Hb + 4O2 = HbO8)

As each O2 molecule binds, it alters the conformation of haemoglobin, making subsequent binding easier (cooperative binding)

  • This means haemoglobin will have a higher affinity for O2 in oxygen-rich areas (like the lung), promoting oxygen loading
  • Conversely, haemoglobin will have a lower affinity for O2 in oxygen-starved areas (like muscles), promoting oxygen unloading

Cooperative Binding of Haemoglobin


Oxygen dissociation curves show the relationship between oxygen levels (as partial pressure) and haemoglobin saturation

  • Because binding potential changes with each additional O2 molecule, the saturation of haemoglobin is not linear

Adult Haemoglobin

  • The oxygen dissociation curve for adult haemoglobin is sigmoidal (i.e. S-shaped) due to cooperative binding
  • There is a low saturation of haemoglobin when oxygen levels are low (haemoglobin releases O2 in hypoxic tissues)
  • There is a high saturation of haemoglobin when oxygen levels are high (haemoglobin binds O2 in oxygen-rich tissues)

Oxygen Dissociation Curve – Adult Haemoglobin

adult dissociation curve

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•  Fetal hemoglobin is different from adult hemoglobin allowing the transfer of oxygen in the placenta onto the

    fetal hemoglobin

Fetal Haemoglobin

  • The haemoglobin of the foetus has a slightly different molecular composition to adult haemoglobin
  • Consequently, it has a higher affinity for oxygen (dissociation curve is shifted to the left)
  • This is important as it means fetal haemoglobin will load oxygen when adult haemoglobin is unloading it (i.e. in the placenta)
  • Following birth, fetal haemoglobin is almost completely replaced by adult haemoglobin (~ 6 months post-natally)
  • Fetal haemoglobin production can be pharmacologically induced in adults to treat diseases such as sickle cell anaemia

Oxygen Dissociation Curve – Fetal Haemoglobin

fetal dissociation curve

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•  Analysis of oxygen dissociation curves for haemoglobin and myoglobin


  • Myoglobin is an oxygen-binding molecule that is found in skeletal muscle tissue
  • It is made of a single polypeptide with only one heme group and hence is not capable of cooperative binding
  • Consequently, the oxygen dissociation curve for myoglobin is not sigmoidal (it is logarithmic)
  • Myoglobin has a higher affinity for oxygen than adult haemoglobin and becomes saturated at lower oxygen levels
  • Myoglobin will hold onto its oxygen supply until levels in the muscles are very low (e.g. during intense physical activity)
  • The delayed release of oxygen helps to slow the onset of anaerobic respiration and lactic acid formation during exercise

Oxygen Dissociation Curve – Myoglobin

myoglobin dissociation curve