Hemoglobin (Hb) is the iron containing coloring matter of red blood cell (RBC). It is a chromoprotein forming 95% of dry weight of RBC and 30% to 34% of wet weight. Function of hemoglobin is to carry the respiratory gases, oxygen and carbon dioxide. It also acts as a buffer. Molecular weight of hemoglobin is 68,000.
NORMAL HEMOGLOBIN CONTENT
Average hemoglobin (Hb) content in
blood is 14 to 16 g/dL. However, the value varies depending upon the age and sex
of the individual.
Age
At birth : 25 g/dL
After 3rd month : 20 g/dL
After 1 year : 17 g/dL
From puberty onwards : 14 to 16 g/dL
At the time of birth, hemoglobin
content is very high because of increased number of RBCs.
Sex
In adult males : 15 g/dL
In adult females : 14.5 g/dL
FUNCTIONS OF HEMOGLOBIN
TRANSPORT OF
RESPIRATORY GASES
Main function of hemoglobin is the
transport of respiratory gases:
1. Oxygen from the lungs to tissues.
2. Carbon dioxide from tissues to
lungs.
1. Transport of
Oxygen
When oxygen binds with hemoglobin, a
physical process called oxygenation occurs, resulting in the formation of oxyhemoglobin.
The iron remains in ferrous state in this compound. Oxyhemoglobin is an
unstable compound and the combination is reversible, i.e. when more oxygen is
available, it combines with hemoglobin and whenever oxygen is required,
hemoglobin can release oxygen
Readily. When oxygen is released from
oxyhemoglobin, it is called reduced hemoglobin or ferrohemoglobin.
2. Transport of
Carbon Dioxide
When carbon dioxide binds with
hemoglobin, carbhemoglobin is formed. It is also an unstable compound
and the combination is reversible,
i.e. the carbon dioxide can be released from this compound. The affinity
ofhemoglobin
for carbon dioxide is 20 times more than that for oxygen.
BUFFER ACTION
Hemoglobin acts as a buffer and plays
an important role in acidbase balance.
STRUCTURE OF HEMOGLOBIN
Hemoglobin is a conjugated protein. It
consists of a protein combined with an ironcontaining
pigment. The protein part is globin
and the ironcontaining pigment is heme. Heme also forms a part of the structure of myoglobin (oxygenbinding pigment
in muscles) and
neuroglobin (oxygenbinding pigment
in brain).
IRON
Normally, it is present in ferrous (Fe2+) form. It is in unstable
or loose form. In some abnormal conditions,
the iron is converted into ferric (Fe3+) state, which is a stable
form.
PORPHYRIN
The pigment part of heme is called
porphyrin. It is formed by four pyrrole rings (tetrapyrrole) called, I, II, III
and IV. The pyrrole rings are attached to one
another by methane (CH4) bridges.
The iron is attached to ‘N’ of each
pyrrole ring and ‘N’of globin molecule.
GLOBIN
Globin contains four polypeptide
chains. Among the four polypeptide chains, two are chains and two are α-chains
Polypeptide chain
Molecula
TYPES OF NORMAL HEMOGLOBIN
Hemoglobin is of two types:
1. Adult hemoglobin – HbA
2. Fetal hemoglobin – HbF
Replacement of fetal hemoglobin by
adult hemoglobin starts immediately after birth. It is completed at about 10 to
12th week after birth. Both the types of hemoglobin
differ from each other structurally
and functionally.
Structural Difference
In adult hemoglobin,
the globin contains two α-chains and two β-chains. In fetal hemoglobin, there
are two α chains and two γ-chains instead of β-chains.
Functional Difference
Functionally, fetal hemoglobin
has more affinity for oxygen
than that of adult hemoglobin. And, the oxygenhemoglobin dissociation curve of
fetal blood is shifted to left.
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