Structure of human hemoglobin.
The α and β subunits are in red and blue.
The iron-containing heme groups in green

Hemoglobin (or haemoglobin) is a protein in red blood cells which contains iron. It is used to transport oxygen around the human body.

Hemoglobin is found in the red blood cells of almost all vertebrates. The exceptions are the fish family Channichthyidae, and the tissues of some invertebrates. It does occur in some invertebrates, but most invertebrates use other chemicals, such as hemocyanin.