Wikipedia: Hemoglobin

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.)

The molecule is mostly protein: mutations in the gene for the haemoglobin protein result in the hereditary diseases sickle cell anaemia and thalassaemia?

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.