What Plasma Does in the Human Body
Plasma is the liquid component of blood that primarily consists of water (90-92%) along with soluble proteins, lipoproteins, extracellular vesicles, and other cellular components, serving as the transport medium for blood cells and essential molecules throughout the body. 1
Primary Composition and Physical Properties
- Plasma constitutes approximately 55% of total blood volume after centrifugation separates it from cellular components 1
- The remaining 10% of plasma (beyond water) contains soluble proteins, lipoproteins, residual platelets, cell fragments (such as red blood cell "ghosts"), and extracellular vesicles 1
- Plasma appears as a clear, transparent yellow fluid under normal conditions 1
Key Physiological Functions
Coagulation and Hemostasis
- Plasma contains fibrinogen, a critical soluble protein that converts into insoluble fibrin polymers during the clotting process 1
- Plasma provides the medium for all coagulation factors (II, VII, IX, X) which require calcium ions and negatively charged phospholipid surfaces to form tenase and prothrombinase complexes 1, 2
- Extracellular vesicles within plasma contribute to cellular homeostasis and hemostasis by providing membrane surfaces for coagulation reactions 1
Transport and Cellular Communication
- Plasma serves as the transport medium for extracellular vesicles (EVs) that facilitate cargo transport between cells, with EVs having a large surface-to-volume ratio (estimated at 1600 mm² per mL of plasma) 1
- The surface of blood-derived EVs is covered by a protein corona that supports their function in cargo transport 1
- Plasma transports lipoproteins (chylomicrons, VLDL, LDL, HDL) at varying concentrations depending on fasting state 1
Immune and Inflammatory Functions
- Plasma-borne extracellular vesicles contribute to complement activation, immunity, and inflammation processes 1
- EVs in plasma are cleared by phagocytosis in multiple organs including lungs, liver, and spleen 1
Clinical Variations and Pathological States
Normal Variations
- Plasma composition varies with donor characteristics including sex (males have higher erythrocyte concentrations), age, circadian rhythm, lifestyle, and medications 1
- In the postprandial state, plasma contains increased lipoprotein concentrations and may appear milky or opaque (lipemic) 1
Abnormal Appearances
- Orange or red plasma indicates hemolysis from disrupted red blood cell membranes releasing hemoglobin, typically from poor venipuncture technique 1
- Turbid, non-transparent plasma indicates clotting has occurred due to absent or insufficient anticoagulation 1
Therapeutic Applications
- Fresh frozen plasma (FFP) serves as replacement therapy for multiple coagulation factor deficiencies and single factor deficiencies when concentrates are unavailable 3
- Plasma products maintain sufficient coagulation protein levels during massive hemorrhage, particularly when prothrombin time and activated partial thromboplastin time exceed 1.5 times normal 4
- Cryoprecipitate maintains fibrinogen levels above 1 g/L during critical bleeding 4