3.8 - Plasma

Question 1

What are the four main fluid compartments in the body?

Answer 1

• intracellular (55% of the fluid) - inside cells
• there are three extracellular fluids:
• interstitial fluid (36%) - found between cells and vital for carrying oxygen and nutrients to cells and transporting away waste products - drains into lymphatic vessels from tissues where it is called lymph
• plasma (7%) - liquid component of blood - very similar to interstitial fluid but more protein
• transcellular fluid (2%) - group of remaining fluids like ocular fluids in eye and cerebrospinal fluid

Question 2

What is plasma?

Answer 2

• the liquid component of blood
• comprises around 55% of blood volume
• if we treated blood with anticoagulant and centrifuged it we would see:
• plasma - 55% of blood volume (top)
• buffy coat - <1% of blood volume - separates plasma and red cells in the tube - consists of leucocytes and platelets
• red cells - 45% of whole blood

Question 3

What is serum?

Answer 3

• plasma minus the clotting factors
• blood is taken into a tube without an anticoagulant, allowed to clot and then centrifuged
• generated by letting blood clot for several minutes, depleting the plasma of coagulation factors and trapping cells and platelets within the clot
• plasma is quicker to prepare, but serum is a cleaner sample (few cells) but takes longer
  Serum separator tubes (SST) have:
• a silica coating which induces clotting
• a layer of gel forming a physical barrier between cells and serum

Question 4

What are the functions of plasma?

Answer 4

• clotting - contains clotting factors and VWF
• immune defence - contains antibodies and complement proteins
• osmotic pressure maintenance - proteins like albumin help to maintain colloidal osmotic pressure
• metabolism - nutrients like glucose, amino acids and vitamins are transported in plasma
• endocrine - many hormones are soluble in plasma and following release into blood, travel to their target organs
• excretion - cell metabolism waste products like urea are transported via plasma to kidneys for removal

Question 5

Plasma proteins

Answer 5

• proteins make up around 7% of plasma
• we can assay them by electrophoresis
• resulting pattern reflects the size and abundance of two major types of plasma protein - serum albumin (55% of plasma proteins) and globulins (35% - alpha1, alpha2, beta1, beta2 and gamma globulins)

Question 6

Serum albumin

Answer 6

• produced by the liver
• several key functions including:
• transport of lipids, hormones and ions
• maintaining osmotic pressure of plasma
• fatty acids released by lipolysis from the breakdown of triglycerides in adipose are transported by albumin for use by tissues in beta oxidation

Question 7

Alpha-1 globulins

Answer 7

• alpha-1 antitrypsin (A1AT) is produced by the liver and enters the circulation
• A1AT plays an important role in inhibiting proteases (in order to protect tissues from other enzymes, notably the neutrophil elastase, released by neutrophils during inflammation)
• defective/deficient A1AT can compromise the lung where degradation of lung tissue leads to a loss of elasticity and respiratory problems

Question 8

Alpha-2 globulins

Answer 8

• haptoglobin binds to haemoglobin released from erythrocytes and the resulting haptoglobin-haemoglobin complex is removed by the spleen
• measuring haptoglobin levels can be informative in the diagnosis of haemolytic anaemia in combination with other parameters
• alpha2-macroglobulin is a broadly active protease inhibitor which can inactivate fibrinolysis (breakdown in fibrin in blood clotting)

Question 9

Beta globulins

Answer 9

• examples include C3 and C4 complement proteins
• transferrin - generated by liver, key role in transportation of dietary iron that is released from ferritin stores

Question 10

Gamma globulins

Answer 10

Main examples:

• immunoglobulins (antibodies)
• C-reactive protein (acute-phase protein)

Diagnostic uses - increases in relative amounts of the gamma fraction of globulins can indicate infection or myeloma

Question 11

What are the sodium and potassium ion levels in the plasma vs RBCs?

Answer 11

• Na+ is the most plentiful cation in the plasma - 30x lower in RBCs
• K+ levels are 30x lower outside RBCs
• the positive charge from intracellular potassium is mainly balanced by extracellular Cl-
• the internally high concentration of K+ inside the cell is also neutralised by a variety of anions e.g. proteins, nucleic acids, phosphorylated proteins

Question 12

How are calcium levels like in cells?

Answer 12

• Ca2+ found at intracellular levels several thousand times lower than outside the cell
• increase in intracellular Ca2+ is associated with signalling (opened Ca2+ channels allows influx from exterior / release of intracellular stores)

Question 13

What is intracellular Mg2+ important for?

Answer 13

Important cofactor of many enzymes

Question 14

How is the balance of electrolyte gradients maintained?

Answer 14

• by the active transport of ions by protein pumps like Na+K+ATPase (ATP hydrolysis needed for energy for transportation)
• for every 3 Na+ out of cell, 2 K+ enter the cell
• more than 1/3 of ATP consumed during rest is used to power the pump
• maintaining concentration gradient is key to functioning of electrically excitable cells e.g. muscle fibres, neurones, and maintenance of cell volume
• ATP levels depleted = cells have a tendency to become more spherical due to inward movement of sodium ions and water

Question 15

What can we use plasma for? - Biomarkers

Answer 15

• plasma and serum provide an easy to obtain clinical specimen that can be subjected to laboratory tests for diagnostic purposes
• the study of plasma proteome and links with disease, medications and lifestyle is useful to inform treatment decisions
• elevated levels of some key molecules provide handy biomarkers of disease

Question 16

What can we use plasma for? - Passive immunotherapy

Answer 16

• plasma contains antibodies
• immunoglobulins found in the gamma globulin fraction can be used to transfer immunity from one individual to another
• intravenous immunoglobulin G (IVIG) - allows introduction of relatively large amounts of IgG, protects against most of the common pathogens donors are exposed to
• hyperimmune globin - donors are screened for high levels/titres of IgG against a particular pathogen, the IgG fraction is isolated from the plasma of these donors and concentrated
• provides passive immunity to a specific pathogen - often given following high-risk exposure to a pathogen
• vaccinations provide active immunity but require days/weeks to elicit a response

Question 17

How could convalescent plasma be used to treat COVID-19 patients?

Answer 17

1. patients infected with SARS-CoV-2 develop antibodies against the virus
2. after the patient has recovered (convalesced), plasma is donated to obtain antibody-rich plasma
3. the plasma is tested for strength (affinity) and number of antibodies
4. if these levels are fine, the plasma is given to a patient infected with COVID-19

• efficacy remains debatable