Hemoglobin (Hb)

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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