Oxygen is carried as Oxyhaemoglobin. Red blood cells contain Haemoglobin, it has a high affinity for oxygen (the tendency to combine with). In the lungs oxygen joins to the iron to form Oxyhaemoglobin. It is a reversible reaction, oxygen will dissociate it near the body cells and return to Haemoglobin. The saturation of Haemoglobin depends on the partial pressure of oxygen. More oxygen dissolved in cells, high 'p' 'p' of oxygen. The affinity of haemoglobin depends on the 'p' 'p' of oxygen.
When there's high 'p' 'p' of oxygen, oxygen will combine with haemoglobin, when there's low 'p' 'p' of oxygen Oxyhaemoglobin will unload oxygen.
When oxygen enters the capillaries at the alveoli, the alveoli have a high 'p' 'p' of oxygen therefore the oxygen loads onto the haemoglobin. When cells respire they use up oxygen low 'p' 'p' of oxygen. Red blood cells deliver the Oxyhaemoglobin to the respiring tissues, where it will unload oxygen.
At the top of the dissociation curve, the 'p' 'p' of oxygen is high (for example in the lungs), haemoglobin has a high affinity for oxygen therefore high percentage saturation.
At the bottom of the dissociation curve, the 'p' 'p' of oxygen is low (for example in respiring tissues) haemoglobin has a low affinity therefore it releases rather than combine, low percentage saturation.
The graph is sigmoid because when the haemoglobin combines with the first oxygen molecule it makes it easier to add the second…