Hemoglobin or haemoglobin (frequently abbreviated as Hb) is the
iron-containing oxygen-transport metalloprotein in the red cells
of the blood in mammals and almost all other vertebrates. Some
invertebrates also employ hemoglobin in oxygen transport, such
as annelid worms, which have the hemoglobin dissolved in the
blood, and the larval aquatic stages of a few insects. Only a
few vertebrates, such as eel larvae and some species of
Antarctic "icefish," have been found to lack hemoglobin.
Hemoglobin is the most efficient oxygen-carrier known. A
protein-metal complex, hemoglobin binds loosely and reversibly
with oxygen under suitable conditions and then releases the gas
under other conditions. In vertebrates, hemoglobin transports
oxygen from the lungs or gills to the rest of the body, such as
to the muscles, where it releases the oxygen load. Structurally,
hemoglobin is a globular protein with an embedded heme (or haem)
group; each heme group contains an iron atom, and this is
responsible for the binding of oxyg