hemoglobin [hemo + L. globus, globe]
The iron-containing pigment of red blood cells that carries oxygen from
the lungs to the tissues. The amount of hemoglobin in the blood
averages 16 g/100 ml in women, 14 to 18 g/100 ml in men. Hemoglobin
is a crystallizable, conjugated protein consisting of heme, an iron-
containing pigment, and globin, a simple protein. In the lungs, 1 g of
hemoglobin combines readily with 1.36 cc of oxygen, by a process
called oxygenation, to form oxyhemoglobin, an unstable compound.
In the tissues where oxygen concentration is low and carbon dioxide
concentration is high [low pH], hemoglobin releases its oxygen.
Hemoglobin also acts as a buffer for the hydrogen ions produced in red
blood cells when carbon dioxide is converted to bicarbonate ions for
transport in the plasma.
When old RBCs are phagocytized by macrophages in the liver, spleen,
and red bone marrow, the iron of hemoglobin is resused immediately to
produce new RBCs or is stored in the liver until needed. The globin is
converted to animo acids for the synthesis of other proteins. The heme
portion is of no further use and is converted to bilirubin, a bile
pigment excreted by the liver in bile.
Hemoglobin combines with carbon monoxide to form the stable compound
carboxyhemoglobin, which renders hemoglobin unable to bond with oxygen
and results in hypoxia of tissues. Oxidation of the ferrous iron or
hemoglobin to the ferric state produces methemoglobin.
Hundreds of different types of hemoglobin have been discovered. Some
of these, such as hemoglobin S, are described in subentries that
follow.
h.Atc...Hemoblogin A that contains a glucose group linked to the
terminal animo acid of the beta chains of the molecule.
h.C disease...A genetic variant of the hemoglobin molecule that causes
a chronic hemolytic anemia.
h.E disease...A genetic variant of hemoglobin that produces a mild form
of hemolytic anemia. It is primarily in persons of SE Asian origin, in
whom it may provide protection against falciparum malaria.
h.H disease...A genetic variant of hemoglobin that causes a chronic
hemolytic anemia.
h.M disorder...A genetic variant of hemoglobin that causes cyanosis and
methemoglobinemia.
h.S disease...A genetic variant of hemoglobin that causes sickle cell
trait in heterozygotes, and sickle cell disease in hemozygotes. It is
common, especially in persons of African ancestry in whom sickle cell
trait is found in 8%-10% of the population.
h.SC disease...A disease of persons who have inherited two abnormal
forms of hemoglobin, S and C. Affected person my have vaso-occlusive
crises similar to those seen in sickle cell anemia, with bony and
viaceral infarcts.