Download Hemoglobin /he·mo·glo·bin/ (he´mo

Hemoglobin Estimation :
Hemoglobin /he·mo·glo·bin/ (he´mo-glo″bin) the oxygencarrying pigment of erythrocytes, formed by developing erythrocytes
in the bone marrow. Many abnormal hemoglobins have been
reported; the first were given capital letters such as hemoglobin E, H,
M, and S, and later ones have been named for the place of discovery.
Homozygosity for hemoglobin S results in sickle cell anemia,
heterozygosity in sickle cell trait. Symbol Hb.
modifications of the Sahli-Hellige method, and 100 percent may be
equal to from 13.8 g to 17.3 g. In the sets usually used in the Navy,
however, 100 percent is equal to 14.5 g of hemoglobin per 100 ml of
whole blood. After reading the percentage on the scale, turn the tube
and read from the other side to get the equivalent reading in grams. If
either scale is hard to read, remember that
100% ÷ 14.5 g = 6.9, so one gram of
hemoglobin is equal to 6.9 percent.
If only one scale can be read, the other reading can be
computed. Caution: Equipment must be clean and dry
before determination is started. Wipe all blood from the outside of
the pipette before you insert it into the tube. Twenty cubic millimeters
is a small volume, and a few blood cells clinging to the outside of the pipette can cause a significant error in findings.
Each erythrocyte contains 200 to 300 molecules of hemoglobin, each
molecule of hemoglobin contains four groups of heme, and each
group of heme can carry one molecule of oxygen. Different kinds of
hemoglobin are identified by their specific combination of polypeptide
chains. Those found most often are alpha and beta chains, with
gamma and delta being found less often. The number of chains of the
different types in the molecule is indicated by subscript numerals and
superscript capital letters. Improved research methods may reveal
many more.
Hb Function
The main functional constituent of the red blood cell, serving as
the oxygen-carrying protein; it is a type of HEMOPROTEIN in which each
molecule is a tetramer composed of four monomers held together by
weak bonds. It consists of two pairs of polypeptide chains, the
GLOBINS, each having an attached HEME molecule composed of iron
plus a PROTOPORPHYRIN molecule. Symbol Hb.
Chemistry and Physiology
The iron atom has a free valence and can bind one molecule of
oxygen. Thus, each hemoglobin molecule can bind one molecule of
oxygen. The binding of oxygen by one monomer increases the affinity
for oxygen of the others in the tetramer. This makes hemoglobin a
more efficient transport protein than a monomeric protein such as
MYOGLOBIN .
Oxygenated hemoglobin (OXYHEMOGLOBIN) is bright red in color;
hemoglobin unbound to oxygen ( DEOXYHEMOGLOBIN) is darker. This
accounts for the bright red color of arterial blood, in which the
hemoglobin is about 97 per cent saturated with oxygen. Venous
blood is darker because it is only about 20 to 70 per cent saturated,
depending on how much oxygen is being used by the tissues. The
affinity of hemoglobin for carbon monoxide is 210 times as strong as
its affinity for oxygen. The complex formed ( CARBOXYHEMOGLOBIN)
cannot transport oxygen. Thus, carbon monoxide poisoning results in
hypoxia and asphyxiation.
Another form of hemoglobin that cannot transport oxygen is
METHEMOGLOBIN, in which the iron atom is oxidized to the +3 oxidation
state. During the 120-day life span of a red blood cell, hemoglobin is
slowly oxidized to methemoglobin. At least four different enzyme
systems can convert methemoglobin back to hemoglobin. When
these are defective or overloaded, METHEMOGLOBINEMIA can result,
with high methemoglobin levels causing dyspnea and cyanosis.
A secondary function of hemoglobin is as part of the blood buffer
system. The histidine residues in the globin chains act as weak bases
to minimize the change in blood pH that occurs as oxygen is
absorbed and carbon dioxide released in the lungs and as oxygen is
delivered and carbon dioxide taken up from the tissues.
As erythrocytes wear out or are damaged, they are ingested by
macrophages of the reticuloendothelial system. The porphyrin ring of
heme is converted to the bile pigment BILIRUBIN, which is excreted by
the liver. The iron is transported to the bone marrow to be
incorporated in the hemoglobin of newly formed erythrocytes.
The hemoglobin concentration of blood varies with the hematocrit.
The normal values for the blood hemoglobin concentration are 13.5 to
18.0 g/100 ml in males and 12.0 to 16.0 g/100 ml in females. The
normal mean corpuscular hemoglobin CONCENTRATION, which is the
concentration within the red blood cells, is 32 to 36 g/100 ml.
VARIANT AND ABNORMAL HEMOGLOBINS. There are six different types of
globin chains, designated by the Greek letters α, β, γ, δ, ε, and δ. The
composition of a hemoglobin is specified by a formula such as α 2β2,
which indicates a tetramer containing two α chains and two β chains.
The chains are coded by different genes, which are turned on and off
during development in order to produce hemoglobins with the
oxygen-carrying properties required at each developmental stage. In
the first three months of embryonic development, when blood cells
are produced in the yolk sac, embryonic hemoglobins such as Hb
Gower (α2Aε2) or Hb Portland (δ2γ2) are produced. As erythropoiesis
shifts to the liver and spleen, the fetal hemoglobin Hb F (α 2γ2)
appears. When erythropoiesis shifts to the bone marrow during the
first year of life, the adult hemoglobins Hb A (α 2β2) and Hb A2 (α2δ2)
begin to be produced.
Many abnormal hemoglobins arising from mutations have been
discovered. Some have altered oxygen affinity, some are unstable,
and in some the iron atom is oxidized, resulting in congenital
METHEMOGLOBINEMIA. Some mutations result in a reduced rate of
hemoglobin synthesis. All such conditions are known as
HEMOGLOBINOPATHIES.
The most common hemoglobinopathy is SICKLE CELL DISEASE, caused
by a mutation replacing the sixth amino acid in the β chain, normally
glutamic acid, by valine. The variant hemoglobin α 2βS2 is known as
Hb S. Mutations resulting in reduced synthesis of one of the chains
are called THALASSEMIAS. They can result from deletion of the gene for
a chain or from a mutation in the regulatory gene that controls the
synthesis of the chain.
The life cycle of red blood cells and the breakdown of hemoglobin.
From Polaski and Tatro, 1996.
hemoglobin A1c hemoglobin A with a glucose group attached to the
amino terminal of the beta chain; it is made at a slow constant rate
during the 120-day life span of the erythrocyte. It accounts for 3 to 6
per cent of the total hemoglobin in a normal person and up to 12 per
cent in persons with diabetes mellitus. Increased levels correlate with
glucose intolerance in diabetics; with good diabetic control its level
returns to normal range, so that periodic assays can be helpful in
evaluating effective control of diabetes.
glycated hemoglobin (glycosylated hemoglobin) any of various
hemoglobins with GLUCOSE attached nonenzymatically; the most
common one is hemoglobin A1c. The percentage of hemoglobin that
is glycosylated can be assessed over a long period of time as a
gauge of blood sugar control; the normal range for a nondiabetic
person is between 4 and 6 per cent.
mean corpuscular hemoglobin (MCH) the average hemoglobin
content of an erythrocyte, conventionally expressed in picograms per
red cell, obtained by multiplying the blood hemoglobin concentration
(in g/dl) by 10 and dividing by the red cell count (in millions per ml):
MCH = Hb/RBC.
hemoglobin, a protein-iron compound in red blood cells that carries
oxygen from the lungs to body cells.
Carcass Hemoglobin Tests
Include hemoglobin extraction test, hemoglobinpseudoperoxidase test. Used on suspect meat to determine if it has
been properly bled out; poor bleeding is an indication of fever or
septicemia.
Hemoglobin Concentration
varies with the hematocrit; determined by several methods. Assesses
the oxygen-carrying capacity of blood.
Cyanotic hemoglobin malformations
insufficient oxygenated hemoglobin is received in the peripheral
capillary beds resulting in blue discoloration of tissues, and an
incapacity of the body to maintain a life-sustaining level of activity.
Glycosylated hemoglobin test
Measurement of the percentage of hemoglobin A molecules
that formed a stable ketoamine linkage between the amino terminal
valine residue of the beta chain and a glucose moiety; used to assess
diabetic control.
Hemoglobin-Oxygen dissociation Curve
The incremental increase in oxygen saturation of the
hemoglobin with each unit increase of the partial pressure of oxygen
in the blood. Any factor that shifts the curve to the right will
automatically reduce the concentration of O 2 held by the hemoglobin
and increase the rate of its delivery to tissues.
Hemoglobin variants
The globin part of hemoglobin is composed of a large number
of amino acids and the hemoglobins are therefore susceptible to a
great many variations. In humans a large number of variants have
been identified but only a few in animals and none are deleterious.
The identified ones are the three adult hemoglobin types, HbA, HbB
and HbC. There is also an embryonic, HbE, and a fetal hemoglobin,
HbF.
At birth up to 30% of the hemoglobin is HbA; most adult Hb is HbA
with small amounts of HbF and HbA 2; Hb defects are inherited and
termed hemoglobinopathies.