A chromoprotein, made up of the protein, globin and the iron-containing pigment HAEM. The main function of haemoglobin is to act as a carrier of oxygen from the lungs to all the tissues of the body.

It exists in two forms: simple haemoglobin (Hgb), found in venous blood; and oxyhaemoglobin, which is a loose compound with oxygen, and is found in arterial blood after coming into contact with air in the lungs. Oxyhaemoglobin is broken down as the blood passes through the tissues, which take up the oxygen for their own use. When the haemoglobin leaves the lungs, it is 97 per cent saturated with oxygen; when it comes back to the lungs in the venous blood, it is 70 per cent saturated. The oxygen content of 100 millilitres of blood leaving the lungs is 19.5 millilitres, and that of venous blood returning to the lungs, 14.5 millilitres. Thus, each 100 millilitres of blood delivers 5 millilitres of oxygen to the tissues of the body. Human male blood normally contains 13–18 grams of haemoglobin per 100 millilitres; in women, there are 12–16 grams per 100 millilitres.