Haemoglobin (or hemoglobin) is the iron-containing oxygen-transport chemical in the red cells of the blood in mammals and other animals. (Some marine organisms use a different compound that contains copper instead of iron.)
At the core of the molecule is a heterocyclic ring, known as a porphyrin which holds an iron atom; this iron atom is responsible for the molecule's oxygen binding properties.
When red cells reach the end of their life, they are broken down, and the haemoglobin molecule broken up and the iron recycled. When the porphyrin ring is broken up, the fragments are normally secreted in the bile by the liver. There is a genetic disorder, known as porphyria in which this mechanism fails to work properly. King George III of England was probably the most famous sufferer from this disease.
See also: myoglobin