Plasma Proteins

When you spin blood in a centrifuge, it separates into three layers:

1. Plasma (the top, straw-colored fluid)
2. Buffy coat (white blood cells + platelets)
3. Red blood cells (at the bottom)

Plasma makes up 55% of total blood volume, and it’s about 90% water. The rest (around 7–8%) is made of plasma proteins, which play vital roles in maintaining life and balance in the body.

Let’s break them down — clearly, deeply, and simply.

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What Are Plasma Proteins?

Plasma proteins are large organic molecules found in the plasma part of the blood. They are mainly synthesized by:

• The liver (majority)
• The plasma cells (lymphocytes) — especially immunoglobulins

These proteins are too large to leave the bloodstream easily, which is why they help maintain osmotic pressure and perform many vital functions.

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Major Classes of Plasma Proteins

There are three main types:

🧬 a. Albumin

• Most abundant plasma protein (~60% of total plasma proteins).
• Made in the liver.
• Molecular weight: ~69,000 Da.

Functions:

1. Maintains colloid osmotic (oncotic) pressure — prevents fluid from leaking out of blood vessels.
2. Transports substances: free fatty acids, bilirubin, calcium, hormones, and drugs.
3. Acts as a buffer to help stabilize blood pH.
4. Serves as a protein reserve — broken down in malnutrition to supply amino acids.

💡 Clinical note: Low albumin = hypoalbuminemia, leading to edema (swelling) due to loss of osmotic balance.

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🧬 b. Globulins

Globulins are a group of diverse proteins with immune and transport roles. They are divided into:

• α1 and α2 globulins
• β globulins
• γ globulins (immunoglobulins)

Functions:

• α and β globulins: transport lipids, metals (iron, copper), and fat-soluble vitamins.
• γ globulins: act as antibodies (IgG, IgA, IgM, IgE, IgD) that defend against pathogens.

💡 Clinical note: Increased γ-globulins are seen in chronic infections or immune responses; decreased levels in immunodeficiency.

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🧬 c. Fibrinogen

• Makes up about 4% of total plasma proteins.
• Synthesized by the liver.
• Converted to fibrin during blood clotting.

Functions:

• Essential for blood coagulation.
• Helps maintain blood viscosity.

💡 Clinical note: Low fibrinogen = bleeding disorders or delayed clotting.

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Minor Plasma Proteins

Other proteins present in smaller amounts include:

• Complement proteins (C1–C9): part of immune defense.
• Lipoproteins: transport cholesterol and triglycerides.
• Enzymes: like alkaline phosphatase, transaminases (diagnostic markers).
• Hormone-binding proteins: carry thyroid, steroid, and sex hormones.

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Functions of Plasma Proteins

Function	Main Protein(s)	Example / Explanation
Osmotic pressure	Albumin	Keeps water inside capillaries
Transport	Albumin, Globulins	Carry hormones, lipids, ions
Immunity	γ-globulins	Antibodies protect against infection
Clotting	Fibrinogen	Converts to fibrin to form clots
Buffering	Albumin	Maintains blood pH
Viscosity	Fibrinogen, Globulins	Affects blood flow and circulation

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Biochemical Note: Plasma vs. Serum

• Plasma: obtained from anticoagulated blood (contains fibrinogen).
• Serum: obtained after clotting (fibrinogen removed).

💡 So, plasma = serum + fibrinogen.

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Clinical Relevance

1. Liver disease: ↓ albumin, ↓ clotting factors.
2. Kidney disease (nephrotic syndrome): albumin lost in urine → edema.
3. Malnutrition: ↓ plasma protein synthesis → swelling, poor healing.
4. Multiple myeloma: abnormal γ-globulin peaks on electrophoresis.

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Quick Recap

• Plasma proteins = Albumin, Globulins, Fibrinogen.
• They maintain osmotic pressure, transport substances, help clotting, and provide immunity.
• Liver is the main site of synthesis (except antibodies).
• Low plasma proteins → edema, poor immunity, bleeding issues.