Define Plasma Proteins. Explain its functions in briel 5 marks Question
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Plasma Proteins — Definition and Functions
Definition
Plasma proteins are a diverse group of proteins dissolved in blood plasma, constituting approximately 7% of plasma volume. They are primarily synthesized in the liver (albumin, fibrinogen, most globulins), with immunoglobulins (γ-globulins) produced by lymphocytes/plasma cells. The three major fractions are:
| Fraction | % of Plasma Proteins | Examples |
|---|---|---|
| Albumin | ~58% | Albumin |
| Globulins | ~37% | α, β, γ-globulins |
| Fibrinogen | ~4% | Fibrinogen |
Normal total plasma protein level: 6–8 g/dL (albumin: 3.5–5.0 g/dL).
Functions of Plasma Proteins
1. Oncotic Pressure (Colloid Osmotic Pressure)
Plasma proteins — primarily albumin — exert an osmotic force of approximately 25 mm Hg across capillary walls. This oncotic pressure draws fluid back from the interstitial space into the blood at the venous end of capillaries, maintaining proper distribution of water between blood and tissues. Loss of plasma proteins (e.g., in kwashiorkor, nephrosis, liver disease) leads to edema.
2. Transport Function
Plasma proteins act as carrier molecules for substances that are otherwise insoluble or unstable in plasma:
- Albumin transports fatty acids, bilirubin, Ca²⁺, hormones, and drugs
- Transferrin (β-globulin) carries iron
- Ceruloplasmin carries copper
- Thyroxine-binding globulin (TBG) transports thyroid hormones
- Haptoglobin binds free hemoglobin
3. Immune Defense
γ-Globulins (Immunoglobulins/Antibodies), secreted by plasma cells, are the primary mediators of humoral immunity. They recognize and neutralize pathogens (bacteria, viruses, toxins) and activate the complement system.
4. Coagulation (Hemostasis)
Fibrinogen is the precursor to fibrin, the structural scaffold of blood clots. Additional coagulation proteins (prothrombin, factors V, VIII, X, etc.) are also plasma proteins synthesized by the liver. Their deficiency leads to bleeding disorders (e.g., hemophilia A = factor VIII deficiency).
5. Buffering Capacity
Plasma proteins contribute approximately 15% of the total buffering capacity of blood. The weak ionization of their –COOH and –NH₂ groups allows them to act as amphoteric buffers, helping maintain blood pH at 7.40. At this pH, proteins are predominantly in the anionic form.
6. Regulatory/Other Functions
- Complement proteins participate in innate immunity and inflammation
- Enzymes and proenzymes (e.g., plasminogen → plasmin for fibrinolysis)
- Contribution to blood viscosity
Clinical Note: Hypoproteinemia (low plasma proteins) occurs in liver disease (↓ synthesis), nephrotic syndrome (urinary loss), and malnutrition (↓ substrate). The hallmark is pitting edema due to reduced oncotic pressure.
Sources: Ganong's Review of Medical Physiology, 26th ed. | Basic Medical Biochemistry: A Clinical Approach, 6th ed. | Junqueira's Basic Histology, 17th ed.
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