• Haemoglobin are protein molecules with a quintenary structure:
  • Primary structure: sequence of amino acids in four polypeptide chains
  • Secondary structure: each of these polypeptide chains is coiled into a helix
  • Tertiary structure: each polypeptide chain is folded into a precise shape
  • Quintenary Structure: All 4 polypeptide chains are linked together to form an almost spherical molecule
  • Each polypeptide chain has a ‘heme group’, each of which containing a ferrous (Fe2+) ion
  • The Obinds to the Fe2+, allowing for 4 O2 molecules to be carried by each haemoglobin.

Loading and Unloading

  • Loading or associating is the binding of oxygen to the haemoglobin
  • Unloading or disassociating is the releasing of oxygen from the haemoglobin
  • Haemoglobin with a high affinity for oxygen takes up oxygen more easily, but releases it less easily
  • Haemoglobin with a low affinity for oxygen takes up oxygen less easily, but releases it more easily
  • The affinity of oxygen changes depending on the pH.
  • At the gas exchange surface, CO2 is constantly being removed, therefore the pH is slightly raised due to the lack of CO2
  • The presence of Carbon Dioxide causes the shape of haemoglobin to change, resulting in its affinity for oxygen to decrease. Oxygen is therefore disassociated. This occurs where there is a high concentration of carbon dioxide, i.e. around the cells, so the oxygen moves from the haemoglobin to the cells.

Oxygen + Haemoglobin ↔ oxyhaemoglobin

Region of the body Oxygen concentration Carbon Dioxide concentration Affinity to Oxygen Result
Gas Exchange Surface High Low High Oxygen is associated
Respiring Tissue low high low Oxygen is disassociated


When the body is at rest, only 1 of the 4 oxygens in the haemoglobin may disassociates as there is a decreased amount of carbon dioxide.

There are different types of haemoglobin which have different affinities to oxygen. Different species have different amino acids sequences, therefore having different tertiary and quaternary protein structures; altering its affinity to oxygen.


Oxygen Dissociation

1st oxygen: the first oxygen takes time due to the shape of the heme groups need to change to allow for the oxygen to bind to it

2nd and 3rd Oxygen: the heme group’s shape has changed, making it easier for the oxygen to bind to it

4th Oxygen: Chance of a successful collision decreases, therefore the 4th oxygen is harder to bind to it


The increased rate of oxygen disassociation the curve shifts to the right of the graph. The saturation of O2 to blood at a given pO2 is lower due to more oxygen being realised; this is the Bohr’s effect.

Foetal Haemoglobin


  • Maternal Haemoglobin will dissociate itself in the placenta and the foetal haemoglobin will load with oxygen
  • The foetus has a higher affinity so it can take oxygen from the lower affinity haemoglobin of the mother


High Altitude Conditions


  • The haemoglobin found in animals in high altitudes have a higher affinity to oxygen due to their being less oxygen pressure available
  • However the oxygen is released less readily.