HEMOGLOBIN-FUNCTIONS, CONCENTRATION AND PRINCIPLES
OF ESTIMATION :-
Hemoglobin (also spelt, haemoglobin) is
present inside the RBCs. It is required for (i) transport of oxygen as well
as (ii) transport of carbon dioxide and (ill) it also behaves as
a blood buffer.
The iron in the Hb is in ferrous state
(Fe++) and even after the combination with oxygen it remains
ferrous. The Hb, in health, after catching oxygen, is called oxyHb (HbO2).
The combination of oxygen and Hb is loose and reversible so that in the
capillaries of the tissues, oxygen can leave the Hb02 and migrate to the
tissues (where the O2 tension is low). Reduced hemoglobin can combine with
oxygen very speedily and that is why, although the transit time of RBC through
a pulmonary capillary may be very short, the uptake of the O2 by Hb is
complete. In some diseased states, the iron of Hb is oxidized and converted
into ferric (Fe+++) iron. The condition is called methemoglobinemia and
oxygen cannot be released easily from such Hb. Normally, the presence of an
enzyme, methemoglobin reductase keeps the iron in ferrous state. Hb also
normally carries C02' It also acts as a blood buffer.
Normal
value
for the concentration of Hb is around 15 gm per 100 ml, but the range is
somewhat wide; for males it is between 14 to 17 gms/l00 ml and for
females between 12 to 16 gms/ 100 ml.
As a bedside method, Hb is commonly
estimated by converting it into acid hematin (Sahli's method) and matching with the
standard. In clinical laboratories, cyanmethemoglobin method is very
popular; in this method, Hb is converted into cyanmethemoglobin and the color
developed is compared against a standard in a suitable colorimeter.
Hemoglobin
chemistry and synthesis. A hemoglobin molecule contains two
ingredients, viz. haem (also spelt as heme) and globin.The haem
is an iron containing compound belonging to the class of compounds called protoporphyrins.
Globin belongs to the class of protein called globulins. Hemoglobin thus is
a conjugated protein.
The globin molecule contains four
polypeptide chains. Two of them have identical amino 'acid number and sequence
and are designated as α chains {each containing 141 amino acids} whereas the
other two, called ß chains have 146 amino acids each and have identical amino
acid sequence. The formula of globin of normal adult hemoglobin, that is HbA,
is written asα2ß2.
With each polypeptide chain, one
molecule of haem is attached. A Hb
molecule, therefore, has four haem molecules. As the iron of each haem
molecule can combine with one molecule (2 atoms) of oxygen, each Hb molecule
can, combination with four molecules of oxygen. The molecular weight of Hb is
64500 { = 64.5 KD+}.
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