Explain how oxygen is loaded, transported and unloaded in red blood cells

Haemoglobin is a molecule in red blood cells with a high affinity for oxygen. It consists of 4 haem groups, each of which associate with one oxygen molecule at high pO₂, typically in the lungs. Haemoglobin dissociates from oxygen in respiring tissues, where there is a low pO₂. Unloading of oxygen is linked to a higher carbon dioxide concentration.