Plasma

Plasma is the liquid extracellular component of blood. It forms the fluid medium in which erythrocytes, leukocytes, and platelets are suspended and transported throughout the cardiovascular system.

Plasma accounts for approximately 55% of total blood volume, although this proportion varies with hydration, hematocrit, physiological state, and disease. It consists predominantly of water, together with proteins, electrolytes, nutrients, hormones, dissolved gases, metabolic products, and other substances carried between tissues and organs.

Plasma is essential not only for transport but also for maintaining blood volume, osmotic balance, acid-base homeostasis, coagulation, immune defense, and the distribution of heat throughout the body.

Composition of Plasma

Water forms the great majority of plasma. The remaining fraction consists of dissolved and suspended substances, particularly plasma proteins and inorganic ions.

Component	General Role
Water	Solvent and transport medium
Plasma proteins	Oncotic pressure, transport, coagulation and immunity
Electrolytes	Osmotic balance, membrane function and acid-base regulation
Nutrients	Supply substrates to tissues
Hormones	Carry endocrine signals
Metabolic wastes	Transport products toward organs of elimination
Dissolved gases	Carry small quantities of oxygen and carbon dioxide

Water

Plasma is approximately 90 to 92% water by mass.

Water acts as the principal solvent for plasma constituents and provides the fluid medium necessary for circulation through blood vessels.

Plasma Proteins

Plasma contains a diverse group of proteins with structural, transport, regulatory, immune, and hemostatic functions.

The major categories include albumin, globulins, and fibrinogen.

Albumin

Albumin is the most abundant plasma protein and is synthesized primarily by hepatocytes in the liver.

It is a major determinant of plasma colloid osmotic, or oncotic, pressure and also serves as an important carrier protein.

Functions of Albumin

• Maintains a substantial portion of plasma oncotic pressure
• Transports fatty acids
• Binds bilirubin
• Transports many drugs
• Binds calcium and other ions
• Transports some hormones and other poorly water-soluble molecules

Globulins

The globulins are a heterogeneous group of plasma proteins traditionally separated electrophoretically into alpha, beta, and gamma fractions.

They include transport proteins, enzymes, complement proteins, and immunoglobulins.

Alpha and Beta Globulins

Many alpha and beta globulins are synthesized in the liver.

They participate in transport, inflammatory responses, complement activity, protease inhibition, and other physiological processes.

Gamma Globulins

The gamma globulin fraction contains many of the circulating immunoglobulins.

Unlike most major plasma proteins, immunoglobulins are produced by plasma cells rather than hepatocytes.

Fibrinogen

Fibrinogen is a soluble plasma protein synthesized by the liver and is essential for normal blood coagulation.

During clot formation, thrombin converts fibrinogen into fibrin, which polymerizes to form a structural network within the blood clot.

Major Plasma Proteins

Protein Group	Major Functions
Albumin	Oncotic pressure and molecular transport
Alpha globulins	Transport, enzyme inhibition and inflammatory functions
Beta globulins	Transport, complement and other functions
Gamma globulins	Antibody-mediated immunity
Fibrinogen	Blood coagulation

Electrolytes

Plasma contains dissolved inorganic ions that are essential for osmotic balance, electrical activity, enzyme function, and acid-base homeostasis.

The principal extracellular cation is sodium, while chloride and bicarbonate are major extracellular anions.

Major Plasma Electrolytes

Electrolyte	General Importance
Sodium	Major extracellular cation and determinant of extracellular fluid osmolality
Chloride	Major extracellular anion and contributor to electroneutrality
Bicarbonate	Major extracellular buffer
Potassium	Important for membrane potentials and cellular function
Calcium	Coagulation, signaling, muscle contraction and other functions
Magnesium	Enzyme activity and cellular processes
Phosphate	Buffering and metabolic functions

Nutrients

Plasma transports nutrients absorbed from the gastrointestinal tract or released from storage tissues.

These include glucose, amino acids, fatty acids, vitamins, minerals, and other molecules required by cells.

Glucose

Glucose circulates dissolved in plasma and serves as an important metabolic fuel.

Its plasma concentration is regulated through coordinated actions of hormones, particularly insulin and glucagon, together with hepatic and peripheral tissue metabolism.

Lipids

Many lipids are poorly soluble in water and therefore circulate in association with proteins or within lipoprotein particles.

Plasma transports triglycerides, cholesterol, phospholipids, and free fatty acids between the intestine, liver, adipose tissue, and other organs.

Amino Acids

Amino acids circulate in plasma after intestinal absorption and release from tissue protein metabolism.

They are transported to tissues for protein synthesis and numerous metabolic pathways.

Hormones

Plasma serves as the principal transport medium for endocrine hormones traveling from their sites of secretion to target tissues.

Water-soluble hormones generally circulate freely, while many lipid-soluble hormones circulate partly bound to plasma proteins.

Metabolic Waste Products

Plasma carries metabolic waste products from tissues toward organs responsible for their processing or elimination.

Examples include urea, uric acid, creatinine, bilirubin, and carbon dioxide-related products.

Dissolved Gases

Small amounts of oxygen and carbon dioxide are transported physically dissolved in plasma.

Most oxygen is carried by hemoglobin within erythrocytes, while carbon dioxide is transported in several forms, including bicarbonate in plasma.

Transport Function

One of the fundamental functions of plasma is the transport of substances throughout the body.

Its continuous circulation connects the gastrointestinal tract, lungs, kidneys, endocrine organs, liver, and peripheral tissues.

Substances Transported by Plasma

• Nutrients
• Hormones
• Electrolytes
• Plasma proteins
• Metabolic waste products
• Drugs and other exogenous compounds
• Heat
• Small quantities of respiratory gases

Plasma Volume

Plasma volume represents the fluid portion of circulating blood volume.

It changes with hydration, hemorrhage, fluid administration, renal function, capillary fluid exchange, and numerous physiological and pathological conditions.

Plasma and Hematocrit

Hematocrit represents the proportion of blood volume occupied primarily by erythrocytes.

Plasma constitutes most of the remaining volume, together with the very small volume occupied by leukocytes and platelets.

Blood after Centrifugation

Layer	Major Components
Upper layer	Plasma
Buffy coat	Leukocytes and platelets
Lower layer	Erythrocytes

Oncotic Pressure

Plasma proteins generate colloid osmotic pressure, commonly called oncotic pressure.

Albumin is the most important contributor because of its abundance and osmotic effect.

Capillary Fluid Exchange

Movement of fluid between plasma and interstitial fluid depends on hydrostatic and osmotic forces across capillary walls, together with properties of the endothelial barrier and lymphatic drainage.

Plasma proteins help retain fluid within the vascular compartment by contributing to oncotic pressure.

Plasma Osmolality

Plasma osmolality reflects the concentration of osmotically active particles in plasma.

Sodium and its accompanying anions are major determinants of extracellular fluid osmolality.

Acid-Base Balance

Plasma participates in maintenance of physiological pH through several buffering systems.

The bicarbonate-carbon dioxide system is particularly important in extracellular fluid.

Bicarbonate Buffer System

Bicarbonate in plasma interacts with carbon dioxide, carbonic acid, pulmonary ventilation, and renal regulation to help stabilize extracellular pH.

This system links the respiratory and renal mechanisms of acid-base homeostasis.

Coagulation

Plasma contains numerous coagulation factors required for hemostasis.

When vascular injury occurs, coordinated activation of these proteins contributes to formation and stabilization of a fibrin-containing clot.

Calcium and Coagulation

Ionized calcium is required for multiple reactions within the coagulation cascade.

Calcium is therefore an important plasma constituent in normal hemostasis.

Immune Function

Plasma contains immunoglobulins, complement proteins, cytokines, acute-phase proteins, and other molecules involved in immune defense and inflammation.

These soluble components interact with circulating leukocytes and tissue immune cells.

Complement Proteins

The complement system consists of circulating plasma proteins that can be activated through several pathways.

Complement contributes to microbial defense, inflammation, opsonization, and other immune processes.

Heat Distribution

The high water content of plasma allows blood to absorb and redistribute heat generated by metabolically active tissues.

Changes in cutaneous blood flow help regulate transfer of heat between the body's core and the external environment.

Plasma Versus Serum

Plasma and serum are not identical. Plasma is the liquid component of anticoagulated blood and retains fibrinogen and other coagulation factors.

Serum is the fluid remaining after blood has been allowed to clot and the clot has been removed.

Feature	Plasma	Serum
Obtained from	Anticoagulated blood	Clotted blood
Fibrinogen	Present	Largely removed during clot formation
Coagulation proteins	Generally retained	Composition altered by clotting
Cells	Removed by centrifugation	Removed with clot and centrifugation

Plasma Collection

To obtain plasma for laboratory analysis, blood is collected into a tube containing an appropriate anticoagulant and then centrifuged.

The cellular components settle, leaving plasma as the upper liquid layer.

Clinical Significance

Plasma Protein Measurement

Measurement of total protein, albumin, and selected globulin fractions can provide information about nutritional status, hepatic protein synthesis, inflammation, renal protein loss, and other physiological or pathological processes.

Hypoalbuminemia

A reduced plasma albumin concentration decreases plasma oncotic pressure and can contribute to movement of fluid into interstitial spaces.

The clinical significance depends on the underlying cause and other factors controlling fluid balance.

Edema

Alterations in plasma oncotic pressure, capillary hydrostatic pressure, endothelial permeability, sodium balance, or lymphatic drainage can contribute to edema.

Plasma protein concentration is therefore one component of the mechanisms governing tissue fluid accumulation.

Dehydration

Loss of body water can reduce plasma volume and concentrate some circulating constituents.

The resulting changes depend on the type and extent of fluid and electrolyte loss.

Hemorrhage

Acute blood loss removes both plasma and formed elements from the circulation.

Subsequent fluid shifts and physiological compensatory mechanisms alter plasma volume and hematocrit over time.

Fresh Frozen Plasma

Fresh frozen plasma is a blood component containing plasma proteins, including multiple coagulation factors.

It is used in selected clinical situations where replacement of multiple coagulation factors is required.

Plasma Exchange

Therapeutic plasma exchange removes a patient's plasma and replaces it with an appropriate replacement fluid.

The procedure can reduce circulating pathogenic antibodies, immune complexes, abnormal proteins, or other plasma constituents in selected diseases.

Laboratory Importance

Plasma is widely used for biochemical and hematological testing.

Measurements of electrolytes, metabolites, proteins, enzymes, hormones, coagulation factors, and many other substances can be performed using plasma samples.

Key Features of Plasma

Feature	Key Point
Type	Liquid extracellular component of blood
Approximate blood volume proportion	About 55%
Major constituent	Water
Major protein	Albumin
Major extracellular cation	Sodium
Major buffer	Bicarbonate system
Coagulation protein	Fibrinogen
Major functions	Transport, volume maintenance, buffering, coagulation and immunity
Difference from serum	Plasma retains fibrinogen and coagulation factors
Cellular relationship	Suspends and transports formed elements of blood

Anatomical and Physiological Importance

Plasma forms the fluid environment of circulating blood and provides the medium through which blood cells, nutrients, signaling molecules, proteins, electrolytes, and metabolic products are transported. Its high water content allows it to function as both a solvent and a mechanism for distributing heat and dissolved substances throughout the body.

Its protein constituents have particularly important roles. Albumin contributes strongly to plasma oncotic pressure and molecular transport, globulins participate in transport and immunity, and fibrinogen contributes to coagulation. Electrolytes and buffering systems within plasma help maintain extracellular osmolality, electrical balance, and physiological pH.

Plasma therefore connects the cardiovascular system functionally with nearly every organ system. It supports circulation, tissue exchange, hemostasis, immune defense, endocrine signaling, waste transport, and maintenance of the internal extracellular environment.