Introduction

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:

1. Coagulants, mainly fibrinogen, aid in blood clotting   
2. Plasma proteins, such as albumin and globulin, that help maintain the colloidal osmotic pressure at about 25 mmHg
3. Electrolytes like sodium, potassium, bicarbonate, chloride, and calcium help maintain blood pH
4. 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]

Cellular Level

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]

Development

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.

In Adults

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. 

Function

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.
• Hormones: hormones are released into the blood and transported to their target organs.
• 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.
• Role in Erythrocyte Sedimentation Rate (ESR): fibrinogen, an acute phase reactant, increases during acute inflammatory conditions and contributes to the increase in ESR, which is used as a diagnostic and prognostic tool.[3]

Related Testing

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:

• Intracellular fluid (ICF): It forms about 28 L (about 40% of total body weight)
• 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]

• Volume = Amount of Tracer/Concentration of Tracer

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. 

Pathophysiology

There are many disease processes associated with plasma:

1. Thrombotic thrombocytopenic purpura (TTP): a type of microangiopathic hemolytic anemia that manifests as fever, thrombocytopenia, hemolytic anemia, renal dysfunction, and neurologic dysfunction. All five criteria might not be present in all patients. It is often due to the deficiency or inhibition of ADAMTS13, a metalloproteinase that breaks apart big von Willebrand factor (vWF) multimers. In TTP, these large vWF multimers are not broken down and cause increased platelet adhesion and thrombosis. Labs will often show anemia, thrombocytopenia, schistocytes on peripheral smear, increased LDH, increased creatinine, and increased bleeding time with normal PT and PTT. Treatment most involves plasma exchange with fresh frozen plasma, steroids, and splenectomy. Platelets should not be given since it causes more thrombosis. Plasma exchange allows for a good prognosis in patients with TTP.[5]
2. Clotting disorders: Deficiency of specific clotting factors cause hemophilia. Hemophilia A is due to factor VIII deficiency, while hemophilia B is due to deficiency of factor IX. Symptoms involve hemarthrosis and intramuscular hematomas. Prophylactic transfusion of a factor VIII- or factor IX-concentrate is the main treatment for children with severe hemophilia; however, this leads over time to the formation of antibodies against these factors.[6]
3. Von Willebrand disease: It is due to a deficiency or abnormal von Willebrand factor (vWF), which is the most common bleeding disorder and is an autosomal dominant disorder. vWF is needed to protect factor VIII, which is critical for secondary hemostasis. The main role of vWF is to allow for platelet-subendothelium interaction and platelet-to-platelet aggregation. The amount of bleeding seen in patients is directly correlated to the severity of deficiency of vWF and factor VIII. Bleeding from mucocutaneous surfaces (gingiva, menstrual bleeding, easy bruising) are commonly seen. Since factor VIII is only mildly reduced, symptoms such as intramuscular hematomas or hemarthrosis are rare. In terms of lab abnormalities, the platelet count is normal, and bleeding time is increased, prothrombin time (PT) is normal, activated partial thromboplastin time (APTT) may increase (depending on the deficiency of Factor VIII). The VWF-ristocetin cofactor activity assay is used for diagnosis. The Von Willebrand ristocetin cofactor assay tests the ability of a plasma sample to agglutinate platelets in the presence of the Ristocetin. The rate of ristocetin-induced agglutination is directly proportional to the amount and activity of the von Willebrand factor.[7]
4. Immunodeficiency: Antibodies or immunoglobulins play a critical role in the immune system to fight off infections. There are 5 classes of immunoglobulins which are IgM, IgG, IgA, IgD, and IgE. The deficiency of each of them can present with unique symptoms. Failure to produce any immunoglobulins occurs in X-linked agammaglobulinemia (Bruton disease), which is due to the failure of pre-B cells to become mature B cells. IgA deficiency is the major mucosal antibody that causes diarrhea and respiratory infections if it is deficient. Hyper IgM syndrome occurs when the inability of CD40 to interact with B cell causes IgM levels to remain high because of the inability to change into other antibody types.[8] 

Clinical Significance

The numerous clinical uses of plasma can be best explained when considering the various forms and components of blood plasma: [9]

1. 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.
2. 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.
3. 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.
4. Albumin: Albumin is the main protein that controls oncotic pressure and serves as the transporter of multiple endogenous and exogenous substances (e.g., drugs) throughout the body. Infusion of albumin is used in the treatment of burns and hemorrhagic shock. Studies have also shown marked improvement in the prognosis of cirrhotic patients.[10] In patients with liver cirrhosis, albumin infusions have decreased mortality in patients with spontaneous bacterial peritonitis and improved outcomes in large volume paracentesis.[11][12] Albumin is also useful in the management of hepatorenal syndrome.
5. Alpha-1 antitrypsin: Alpha-1 antitrypsin is produced in the liver and plays an important role in the lungs by increasing proteases which counteract the effect of elastases produced by the neutrophils in response to inflammation such as smoking. Alpha-1 antitrypsin deficiency is an inherited disorder that could result in emphysema and cirrhosis in early adulthood. Recent advances in treatment have shown success in decreasing the mortality and frequency of exacerbations when human plasma-derived alpha-1 antitrypsin is given intravenously once a week.[13][14]
6. Plasma as a laboratory test: Plasma testing can diagnose and confirm diseases like diabetes based on serum glucose levels or von Willebrand disease. Monitoring of international normalized ratio (INR) in patients on anticoagulants requires serial measurements of plasma prothrombin levels.[15]
7. Plasmapheresis: Plasmapheresis is an effective temporary treatment in many autoimmune diseases. In therapeutic plasmapheresis, the patient’s venous blood is withdrawn, blood cells are separated, and a replacement colloid solution and blood cells are infused in its place.[16] A 4% to 5% human serum albumin solution in saline is the preferred replacement solution in most cases. The following are common conditions where plasmapheresis is utilized:

• Myasthenia gravis
• Chronic inflammatory demyelinating polyneuropathy
• Hyperviscosity in monoclonal gammopathies
• Thrombotic thrombocytopenic purpura
• Guillain-Barre syndrome
• Lambert-Eaton syndrome
• Multiple sclerosis

     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.[17] 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. It has an exorbitantly high impact on the healthcare system, financial resources, and overall disability both in the United States and worldwide.[18][19][20] 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.[21]