1. Carriage of Oxygen Describe the role of haemoglobin in carrying oxygen
Explain the differences between adult and fetal haemoglobin

2. Haemoglobin
Oxygen is transported in the red blood cells (erythrocytes)
These contain the protein haemoglobin
When haemoglobin takes up oxygen it becomes oxyhaemoglobin
Haemoglobin + oxygen  oxyhaemoglobin

3. Haemoglobin Structure
A protein with 4 subunits
Each subunit is made of a polypeptide chain (a protein) and a haem group (non-protein)
The haem group has a single iron atom Fe2+ that can hold an oxygen molecule
The haem group is said to have an attraction (affinity) for oxygen
Each haemoglobin molecule can hold 4 oxygen molecules (one in each subunit)

4. Taking up and Releasing oxygen
Oxygen is absorbed at the lungs as there is a steep diffusion gradient present
Oxygen is released (dissociated) at respiring cells

5. Haemoglobin and oxygen transport
The ability of haemoglobin to take up and release oxygen depends on the amount of oxygen present (the partial pressure or pO2) It is also called the oxygen tension, and is measured in units of pressure (kPa)

6. Dissociation curve
You would expect that if you doubles the amount of oxygen, the amount taken up by haemoglobin would also double, however it does not behave like this.
Saturation of haemoglobin
Concentration (partial pressure) of oxygen

7. S- shaped curve
The oxyhaemoglobin dissociation curve is the s-shaped curve that shows how saturated (full up) the haemoglobin molecules are at different concentrations (partial pressures) of oxygen
Once the majority of haemoglobin molecules have 3 oxygen molecules, it is more difficult for the 4th to get in. This is why it is difficult to get 100% saturation of haemoglobin, even when the oxygen concentration is really high (like in the human lungs)
Saturation of haemoglobin
Concentration (partial pressure) of oxygen
As the oxygen tension (amount) rises, more haemoglobin molecules have gained their first oxygen, meaning that it is now easier to receive their second etc. (a conformational change) so the saturation of the molecules as a whole rapidly increases
Fetal haemoglobin absorbs oxygen from the mothers blood fluid, which reduces the concentration of oxygen making the mothers haemoglobin release some also
At low oxygen concentrations, most of the haem groups that attract the oxygen molecules are in the centre of the haemoglobin molecule, so less are fully saturated (full with oxygen)