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PLASMA PROTEINS

(University Exam Answer - 1st Year Physiology)

Definition

Plasma proteins are the proteins present in the fluid part of the blood (plasma). They are large molecules that normally stay inside the blood vessels and cannot freely pass through capillary walls.

Total plasma protein concentration: 7 to 7.5 g/dL
Plasma proteins remain in the blood and help maintain its normal composition.

Types of Plasma Proteins (Classification)

Plasma proteins are divided into 3 main fractions - seen clearly on electrophoresis:

Fraction	Normal Level	Examples
Albumin	3.5 - 5.0 g/dL	Albumin
Globulins	2.3 - 3.5 g/dL	α1, α2, β, γ globulins
Fibrinogen	0.2 - 0.4 g/dL	Fibrinogen

A/G Ratio (Albumin : Globulin ratio) = normally 1.2 : 1 to 1.5 : 1

Site of Synthesis (Origin)

This is a very common exam question!

Liver synthesizes: Albumin, Fibrinogen, α and β globulins, clotting factors, complement proteins (70-80% of all plasma proteins are made in the liver)
Lymphocytes / Lymphoid tissue synthesizes: γ-globulins (immunoglobulins / antibodies)
Vascular endothelium: von Willebrand factor

Simple rule to remember: "Liver makes all except γ-globulins"

Functions of Plasma Proteins

1. Maintenance of Oncotic Pressure (Most Important Function)

Plasma proteins, mainly albumin, create an osmotic pressure of about 25 mm Hg across the capillary wall
This is called colloid osmotic pressure or oncotic pressure
It pulls water back from tissues into the blood
This prevents fluid from leaking out of blood vessels (prevents edema)

2. Transport / Carrier Function

Many substances are transported in the blood attached to plasma proteins:
- Albumin carries: fatty acids, bilirubin, drugs, calcium, hormones
- Transferrin (β-globulin) carries: iron
- Ceruloplasmin carries: copper
- Haptoglobin carries: hemoglobin
- Thyroxine-binding globulin (TBG) carries: thyroid hormones
- Transcortin (CBG) carries: cortisol

3. Buffering Function

Plasma proteins contribute about 15% of the total blood buffering capacity
They act as buffers because of their free -COOH (acidic) and -NH₂ (basic) groups
At normal blood pH 7.4, proteins are mostly in the anionic (negatively charged) form

4. Defense / Immunity

Gamma globulins (Immunoglobulins) - IgG, IgM, IgA, IgE, IgD - are antibodies that protect the body against infection
Complement proteins help in immune reactions

5. Blood Clotting (Coagulation)

Fibrinogen is converted to fibrin during clotting, which forms the clot structure
All clotting factors (except factor VIII) are synthesized in the liver
Clotting factors are proteins (globulin fractions)

6. Reserve Amino Acid Source

When body tissues become depleted of proteins, plasma proteins can be broken down and used as a source of amino acids
Tissue macrophages can take in plasma proteins by pinocytosis, break them down, and release amino acids for tissue use

7. Viscosity of Blood

Plasma proteins help maintain normal blood viscosity
They also influence erythrocyte sedimentation rate (ESR)

Plasma Protein Electrophoresis

Electrophoresis is a technique used to separate plasma proteins based on their size and electrical charge using an electric field.

Steps:

Serum is applied to cellulose acetate strip
Electric current is passed through buffer
Proteins migrate toward the positive pole (anode) at different speeds
Staining shows separate bands
Densitometer scanning shows relative amounts

Order from anode (+) to cathode (-):

Albumin → α1 → α2 → β → γ (slowest)

Albumin migrates the fastest (largest peak) because it is the most abundant and moves fastest toward the positive pole.

Hypoproteinemia (Decreased Plasma Proteins)

Definition: Decreased plasma protein levels

Causes:

Decreased synthesis:
- Liver disease (cirrhosis, hepatitis) - liver cannot make enough proteins
- Malnutrition / starvation - not enough amino acids
Increased loss:
- Nephrotic syndrome - albumin lost in urine
- Burns - protein lost from burned skin
Malabsorption syndromes - intestinal disease

Effect: Decreased oncotic pressure → water leaks into tissues → EDEMA

Acute Phase Proteins

In inflammation or injury, the liver produces more of certain proteins called acute phase proteins (positive acute phase reactants)
Examples: C-reactive protein (CRP), fibrinogen, alpha-1 antitrypsin, haptoglobin, ceruloplasmin
Albumin is a negative acute phase reactant - its level decreases during inflammation

Important Individual Proteins - Quick Summary

Protein	Main Function	Clinical Note
Albumin	Oncotic pressure, transport	Decreased in liver disease, nephrotic syndrome
Fibrinogen	Blood clotting	Converted to fibrin; absent in afibrinogenemia
Immunoglobulins (γ-globulins)	Antibodies, immunity	Increased in infections/myeloma
Transferrin	Iron transport	Decreased in iron deficiency anemia
Ceruloplasmin	Copper transport	Decreased in Wilson's disease
Haptoglobin	Binds free Hb	Decreased in hemolytic anemia
Alpha-1 antitrypsin	Protease inhibitor	Deficiency causes lung and liver disease
C-reactive protein (CRP)	Acute phase protein	Marker of inflammation

Key Points to Remember for Exam

Total plasma proteins = 7 - 7.5 g/dL
Albumin is the most abundant plasma protein (3.5-5 g/dL)
Liver is the main site of synthesis (makes 70-80%)
γ-globulins are made by lymphocytes (not liver)
Oncotic pressure = 25 mm Hg (mainly due to albumin)
Decreased protein → decreased oncotic pressure → edema
Electrophoresis order: Albumin, α1, α2, β, γ
A/G ratio = normally 1.2-1.5 : 1 (reversal seen in liver disease, myeloma)

Sources: Ganong's Review of Medical Physiology (26th Ed.), Guyton & Hall Textbook of Medical Physiology, Harper's Illustrated Biochemistry (32nd Ed.), Medical Physiology (Boron & Boulpaep)

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