Plasma, also known as blood plasma, appears light-yellowish or straw-colored. It serves as the liquid base for whole blood. Whole blood minus erythrocytes (RBCs), leukocytes (WBCs), and thrombocytes (platelets) make up the plasma. Serum, sometimes mistakenly considered synonymous with plasma, consists of plasma without fibrinogen. Plasma contains 91% to 92% of water and 8% to 9% of solids. It mainly comprises of:
Extraction of Plasma
It can be separated from whole blood by the process of centrifugation, i.e., spinning whole blood with an anticoagulant in a centrifuge. Plasma is lighter, so it forms the upper yellowish layer, while the denser blood cells fall to the bottom. The plasma collected is frozen within 24 hours to preserve the functionality of the various clotting factors and immunoglobulins; it is thawed before use and has a shelf life of 1 year. Interestingly, while O- is the preferred universal donor for blood, the plasma of AB blood groups are the most preferred because their plasma does not contain antibodies, making it acceptable for everyone without fear of an adverse reaction.
Plasma, like whole blood, is initially tested, to ensure the safety of recipients. As per the FDA regulations, the collected plasma undergoes a battery of tests to identify transmittable diseases, mainly hepatitis A, B, and C along with syphilis and HIV. The process of fractionation separates individual plasma proteins. 
The specific gravity of plasma is 1.022 to 1.026 as compared to the specific gravity of blood which is 1.052 to 1.061. Plasma forms 55% and red blood cells form 45% of the total blood. Four major products derived from plasma which can be used are fresh-frozen plasma (FFP), plasma frozen within 24 hours of phlebotomy (FP24), cryoprecipitate-poor plasma (CPP), and thawed plasma. FP24, CPP, and thawed plasma contain varying amounts of clotting factors. 
Plasma proteins, on the other hand, have distinct organs that produce them based on an individual's stage of development.
In the embryonic stage, the mesenchymal cells are responsible for the plasma cell production. The first protein to be synthesized is albumin, followed by globulin and the other plasma proteins.
The reticuloendothelial cells of the liver are in charge of plasma protein synthesis in adults. The bone marrow, degenerating blood cells, and general body tissue cells along with the spleen also contribute to the formation of plasma proteins. Gamma globulins originate from B lymphocytes, which in turn form immunoglobulins.
The origin of plasma, which constitutes 55% of total blood, is interesting because no organ produces it. Instead, it is formed from water and salts absorbed through the digestive tract.
As plasma forms the liquid base of blood, the functions carried out by plasma and blood overlap. The multitude of functions include:
Water constitutes about two-thirds of the human body. In an adult man weighing 70 kg, the body water content is about 42L. This water content is divided into two major compartments:
Plasma can be measured by the use of marker substances like radioactive iodine (131 I) and Evans blue (T-1824). Evans blue is the commonly used marker substance (aka tracer) since it binds strongly with albumin. The concept behind using a tracer is to use one that is well distributed in the compartment of interest. A known amount of tracer is introduced into the compartment, and its volume of distribution is measured. 
Volume = Amount of tracer/Concentration of tracer
Compartment volumes are measured based on the volume of distribution of tracer. In the case of measuring plasma volume, the albumin-bound tracer, i.e., Evans blue, is used. As albumin tends to continuously leak out of the circulation, the tracer concentration is measured at serial intervals and is plotted on a logarithmic curve. This curve is then extrapolated to identify a “zero time” that allows the estimation of a virtual volume of distribution. The volume of distribution measured is the volume of plasma.
There are many disease processes associated with plasma:
The numerous clinical uses of plasma can be best explained when considering the various forms and components of blood plasma: 
8. Platelet-rich Plasma (PRP): PRP is defined as autologous blood with a concentration of platelets above baseline reference values. Traditionally, PRP injections have been used over the last three decades in maxillofacial and plastic surgery. More recently, its use throughout orthopedics and sports medicine has been well-established and heavily controversial . The use of PRP injections in the setting of acute or acute-on-chronic musculoskeletal pathology continues to remain debated. One of the more heavily debated areas regarding PRP use is in the management of moderate knee osteoarthritis. Knee osteoarthritis afflicts a significant portion of the adult population and has an exorbitantly high impact on the healthcare system, financial resources, and overall disability both in the United States and worldwide.  A recent level I study investigating nearly 200 patients randomized between 3 groups (sham control, hyaluronic acid injections, and leukocyte-poor PRP injections) demonstrated superior patient-reported pain and functional outcome scores at 12-month follow-up in patients managed with PRP injections as opposed to the sham control injection group (normal saline only) and those managed with hyaluronic acid injections. 
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