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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:
Plasma proteins, such as albumin and globulin, that help maintain the colloidal osmotic pressure at about 25 mmHg
Electrolytes like sodium, potassium, bicarbonate, chloride, and calcium help maintain blood pH
Immunoglobulins help fight infection and various other small amounts of enzymes, hormones, and vitamins
Issues of Concern
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, forming 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 is 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.<1>
The specific gravity of plasma is 1.022 to 1.026 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 the 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.<2>
Plasma proteins, on the other hand, have distinct organs that produce them based on an individual"s stage of development. In Embryo
In the embryonic stage, the mesenchymal cells are responsible for 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, general body tissue cells, and the spleen also contribute to the formation of plasma proteins. Gamma globulins originate from B lymphocytes, which in turn form immunoglobulins.
Organ Systems Involved
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:
Coagulation: fibrinogen plays a major role in blood clotting along with other procoagulants like thrombin and factor X.
Defense: immunoglobulins and antibodies in plasma play an important role in the body’s defense against bacteria, viruses, fungi, and parasites.
Maintenance of Osmotic Pressure: the colloidal osmotic pressure is maintained at around 25 mmHg by the plasma proteins like albumin synthesized by the liver.
Nutrition: transportation of nutrients like glucose, amino acids, lipids, and vitamins absorbed from the digestive tract to different parts of the body act as a source of fuel for growth and development.
Respiration: transportation of respiratory gases, i.e., carrying oxygen to the various organs and carrying carbon dioxide back to the lungs for excretion.
Excretion: the blood removes nitrogenous waste products produced after cellular metabolism and transports them to the kidney, lungs, and skin for excretion.
Regulation of Acid-Base Balance: plasma proteins contribute to acid-base balance through their buffering action.
Regulation of Body Temperature: this is maintained by balancing heat loss and heat gain in the body.
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:
Extracellular fluid (ECF): It forms about 14 L (about 20% of total body weight), of which 15% is interstitial fluid and 5% is plasma
Plasma can be measured using 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. <4>
Compartment volumes are measured based on the volume of distribution of tracer. When 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.
The numerous clinical uses of plasma can be best explained when considering the various forms and components of blood plasma: <9>
Whole Plasma: Fresh frozen plasma is indicated in the treatment of massive bleeds resulting in shock, in disseminated intravascular coagulation, burns, and liver disease—the coagulants found in plasma aid in decreasing bleeding time and stabilizes the patient. Fresh frozen plasma also plays an important role as an immediate and effective antidote for warfarin reversal. The first-line treatment of thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS) is plasma exchange with 40 mL of plasma per kg body weight. In neonates, plasma plays a role in the exchange transfusion of plasma of neonates with severe hemolysis or hyperbilirubinemia. Plasma is also utilized in filling the oxygenator in extracorporeal membrane oxygenation in neonates.
Clotting factors: Clotting factors and von Willebrand factor (vWF) found in plasma play an essential role in blood clotting and are activated by damage to the endothelium resulting in exposure of collagen found beneath the endothelium of the blood vessel. People with blood clotting deficiencies such as hemophilia and von Willebrand disease can suffer massive internal bleeds with a minor injury. Such patients benefit greatly from plasma protein derivatives such as factor VIII concentrate and factor IX concentrate.
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Immunoglobulins: Immunoglobulins protect the body against invading bacteria and viruses and play a key role in the body’s defense. Certain immunological disorders like congenital or acquired primary immune deficiency occur when the body cannot produce antibodies or experience the adverse effects of cancer treatments that harm the antibodies. Both disorders benefit greatly from immunoglobulin infusions. Immunoglobulins also play a major role in passive immunization. Antidotes to diseases such as chickenpox, rabies, hepatitis, and tetanus are the initial treatment after suspected exposure to limit disease progression. Such specific immunoglobulins are derived when patients who have been previously affected by a disease donate plasma, for example, chickenpox. This plasma contains high amounts of circulating antibodies against chickenpox that can be collected and stored after fractionation for use as post-exposure vaccines for varicella.