Red blood cells contain haemoglobin, Haemoglobin is a large protein with a quaternary structure that contains a haem group which contains iron and gives haemoglobin its red colour. Haemoglobin has a high affinity for oxygen, meaning it readily picks up oxygen, and each molecule can carry four oxygen molecules. In the lungs, oxygen joins to the iron in the haemoglobin to form oxyhaemoglobin. This is a reversible reaction so when the oxygen dissociates from the oxyhaemoglobin, turning it back to haemoglobin.
Haemoglobin saturation depends on the partial pressure of oxygen. The partial pressure is a measure of the oxygen concentration in the blood. The greater the concentration, the higher the partial pressure. Haemoglobins affinity for oxygen varies depending on the partial pressure of oxygen. Oxygen enters blood capillaries at the alveoli in the lungs. Alveoli have a high partial pressure so the oxygen in the alveoil loads onto haemoglobin to form oxyhaemoglobin easily. When cells respire, they use up oxygen and this lowers the partial pressure of oxygen. Red blood cells deliver oxyhaemoglobin to respiring tissues, where it unloads its oxygen, the haemoglobin then returns to the lungs to pick up more oxygen.
A dissociation curve shows how saturated the haemoglobin is with oxygen at any given partial pressure. A dissociation graph is s shaped because when haemoglobin conbines with the first oxygen molecule, its shape alters in a way that makes it easier for the other molecules to join aswell. However, as the haemoglobin starts to become saturated, it gets harder for more oxygen molecules to join and as a result, the curve has a steep bit in the middle where it is really easy for oxygen molecules to join and shallow bits at each end where it is harder. When the curve is steep, a small change in partial pressure causes a big change in the amount of oxygen carried by haemoglobin.
Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. Fetal haemoglobin has a higher affintity for oxygen meaning the fetus's blood is better at absorbing oxygen than its mothers blood.