PROPERTIES OF PLASMA PROTEINS

PROPERTIES OF PLASMA PROTEINS

MOLECULAR WEIGHT

Albumin : 69,000

Globulin : 1,56,000

Fibrinogen : 4,00,000

Thus, the molecular weight of fibrinogen is greater

than that of other two proteins.

ONCOTIC PRESSURE

Plasma proteins are responsible for the oncotic or osmotic pressure in the blood. Osmotic pressure exerted by proteins in the plasma is called colloidal osmotic (oncotic) pressure (Chapter 3). Normally, it is about 25 mm Hg. Albumin plays a major role in exerting oncotic pressure.

SPECIFIC GRAVITY

Specific gravity of the plasma proteins is 1.026.

BUFFER ACTION

Acceptance of hydrogen ions is called buffer action. The plasma proteins have 1/6 of total buffering action of the blood.

ORIGIN OF PLASMA PROTEINS

IN EMBRYO

In embryonic stage, the plasma proteins are synthesized by the mesenchyme cells. The albumin is synthesized first and other proteins are synthesized later.

IN ADULTS

In adults, the plasma proteins are synthesized mainly from reticuloendothelial cells of liver. The plasma

proteins are synthesized also from spleen, bone marrow, disintegrating blood cells and general tissue cells. Gamma globulin is synthesized from B lymphocytes.

FUNCTIONS OF PLASMA PROTEINS

Plasma proteins are very essential for the body. Following are the functions of plasma proteins:

1. ROLE IN COAGULATION OF BLOOD

Fibrinogen is essential for the coagulation of blood.

2. ROLE IN DEFENSE MECHANISM OF BODY

Gamma globulins play an important role in the defense mechanism of the body by acting as antibodies (immune substances). These proteins are also called immunoglobulins. Antibodies react with antigens

of various microorganisms, which cause diseases like diphtheria, typhoid, streptococcal infections, mumps, influenza, measles, hepatitis, rubella, polio myelitis, etc.

3. ROLE IN TRANSPORT MECHANISM

Plasma proteins are essential for the transport of various substances in the blood. Albumin, alpha globulin and beta globulin are responsible for the transport of the hormones, enzymes, etc. The alpha and beta globulins play an important role in the transport of metals in the blood.

4. ROLE IN MAINTENANCE OF OSMOTIC PRESSURE IN BLOOD

At the capillary level, most of the substances are exchanged between the blood and the tissues. However, autobecause of their large size, the plasma proteins cannot pass through the capillary membrane easily and remain in the blood. In the blood, these proteins exert the colloidal osmotic (oncotic) pressure. Osmotic pressure exerted by the plasma proteins is about 25 mm Hg. Since the concentration of albumin is more than the other plasma proteins, it exerts maximum pressure.

Globulin is the next and fibrinogen exerts least pressure.

Importance of Osmotic Pressure – Starling’s Hypothesis

Osmotic pressure exerted by the plasma proteins plays an important role in the exchange of various substances between blood and the cells through capillary membrane. According to Starling’s hypothesis, the net filtration through capillary membrane is proportional to the hydrostatic pressure difference across the membrane minus the oncotic pressure difference.

5. ROLE IN REGULATION OF ACID-BASE BALANCE

Plasma proteins, particularly the albumin, play an important role in regulating the acidbase

balance in the blood. This is because of the virtue of their buffering action. Plasma proteins are responsible for 15% of the buffering capacity of blood.

6. ROLE IN VISCOSITY OF BLOOD

Plasma proteins provide viscosity to the blood, which is important to maintain the blood pressure. Albumin

provides maximum viscosity than the other plasma proteins.

7. ROLE IN ERYTHROCYTE SEDIMENTATION RATE

Globulin and fibrinogen accelerate the tendency of rouleaux formation by the red blood cells. Rouleaux

formation is responsible for ESR, which is an important diagnostic and prognostic tool .

8. ROLE IN SUSPENSION STABILITY OF RED BLOOD CELLS

During circulation, the red blood cells remain suspended uniformly in the blood. This property of the red blood cells is called the suspension stability. Globulin and fibrinogen help in the suspension stability of the red blood cells.

9. ROLE IN PRODUCTION OF TREPHONE SUBSTANCES

Trephone substances are necessary for nourishment of tissue cells in culture. These substances are produced by leukocytes from the plasma proteins.

10. ROLE AS RESERVE PROTEINS

During fasting, inadequate food intake or inadequate protein intake, the plasma proteins are utilized by the body tissues as the last source of energy. Plasma proteins are split into amino acids by the tissue macrophages. Amino acids are taken back by blood and distributed throughout the body to form cellular protein molecules. Because of this, the plasma proteins are called the reserve proteins.

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