Describe the loading and unloading of oxygen by Haemoglobin in red blood vessels

Haemoglobin is a protein consisting of four haem groups contained in four polypeptide chains. The loading of oxygen consists of oxygen binding to iron in the haem group at areas of higher oxygen concentration such as in the lungs, this is also known as association. The unloading of oxygen is where oxygen unbinds at areas of lower oxygen concentration such as at respiring cells and this is dissociation. When haemoglobin is bound to oxygen, it exists as oxyhaemoglobin.