Question 16

College Answer

This question was generally well answered. The constituents of plasma include water, electrolytes, glucose, liver enzymes, urea, creatinine, uric acid, dissolved gases and proteins. Plasma does not contain any cells. The proteins in plasma are albumin, globulins and fibrinogen. The globulins include alpha 1 and 2 and beta globulins and gamma globulins. Examples of α1-Globulins are: α1-fetoprotein, α1-protease inhibitor and prothrombin. 
Examples of α2-Globulins include: ceruloplasmin, haptoglobin, α2-macroglobulin and thyroxin-binding globulin. Examples of β-Globulins are: C-reactive protein, β2-microglobulin and transferrin. Examples of δ-Globulins are the immunoglobulins , IgG, IgA, IgM, etc. There are many more other globulins including the coagulation factors, the complement system and lipoproteins. The functions of plasma proteins include oncotic pressure, 
transport/carrier function, role in acid base balance (buffering, CO2 transport) and proteolytic systems such as complement, kinins, coagulation and fibrinolysis. More functions include the immune response, enzyme activity eg pseudocholinesterase, metabolism i.e. plasma proteins can be broken down and contribute amino acids to the 
amino acid pool and a role in thermoregulation. Many answers were deficient in details on the plasma proteins and their functions. The question asked to “list” the constituents, so the level of detail required to score marks reflected this and should have been achievable in the allocated timeframe.

Discussion

Plasma  is the cell-free liquid component of blood

• 55% of the total blood volume (the rest is cells, 45%); ~3.5L in an average person
• A pale straw coloured fluid with a specific gravity of around 1.022

Constituents of plasma:

• Water: 90% by volume
• Gases: Nitrogen, carbon dioxide, oxygen, traces of nitric oxide and hydrogen sulfide
• Electrolytes: Sodium, chloride, bicarbonate, potassium, magnesium, calcium
• Low molecular weight nonelectrolytes: ​​​​​​Glucose, fatty acids, amino acids, vitamins, hormones, urea
• High molecular weight molecules: Blood proteins of numerous varieties 

Plasma proteins: 70-90g/L, i.e. 350-450g in an average person, of which 80% is albumin

• Albumin
• Globulins
  • α-1 globulins (α1-foetoprotein, α1-antitrypsin, α-lipoproteins)
  • α-2 globulins (ceruloplasmin, haptoglobin, plasminogen, thyroxin-binding globulin)
  • β-globulins (β-lipoproteins, CRP, transferrin, β2-microglobulin)
  • γ-globulins (immunoglobulins)​​​​​​​
• Fibrinogen (a glycoprotein)

Function of plasma proteins:

• Primary role in the plasma:
  • Oncotic function​​​​​​​: albumin maintains oncotic pressure at ~ 25 mmHg 
  • Coagulation: fibrinogen and the clotting cascade enzymes
  • Buffering: proteins contribute about 20% of the non-bicarbonate buffering
• Immune function: immunoglobulins and complement
• Signalling function: endocrine and paracrine hormones, eg. insulin
• Transport function:
  • Transport of macronutrients, eg. lipoproteins
  • Transport of micronutrients, eg. transferrin
  • Transport of hormones, eg. thyroid-binding globulin
• Non-physiological functions of clinical value:
  • ​​​​​​​Drug binding, eg. albumin and α-1 acid glycoprotein 
  • Biomarker,  eg. myoglobin, procalcitonin, CRP, light chains

Mathew, Joscilin, and Matthew Varacallo. "Physiology, blood plasma." (2018).

Benjamin, Richard John, and Lisa Swinton McLaughlin. "Plasma components: properties, differences, and uses." Transfusion 52 (2012): 9S-19S.

Schaller, Johann, et al. Human blood plasma proteins: structure and function. John Wiley & Sons, 2008.

Anderson, N. Leigh, and Norman G. Anderson. "The human plasma proteome: history, character, and diagnostic prospects." Molecular & cellular proteomics 1.11 (2002): 845-867.