Q1iii / 19B11: Constituents of plasma + function of albumin
19B11: Exam Report
Outline the composition of plasma (50% of marks). Describe the functions of albumin (50% of marks).
30% of candidates passed this question.
A good answer began with a definition of plasma and then listed the components – water, albumin, globulins, fibrinogen and other proteins before mentioning the lipid content, nutrient content, wastes and electrolytes. Frequently the breakdown of the globulin component was inaccurate. A common omission was dissolved gas components. Descriptions of the calculation of oncotic pressure and GFR were not asked and hence did not attract marks.
The functions of albumin may be subdivided into: Osmotic pressure, transport function, acid-base buffer, anti-oxidant, anticoagulant effect, protein store, metabolism and ‘other’.
Q1iii / 19B11: Outline the composition of plasma (50 marks). Describe the functions of albumin (50 marks)
Definition & Normal Values
- Plasma = the fluid medium of the intravascular compartment which transports substances between tissues
- Plasma ≈ 40mL/kg
NB: Serum = whole blood which has settled & clot removed
i.e. Plasma – (clotting factors + fibrinogen)
Plasma Components
- Water → 94% of plasma is H2O
- Electrolytes
- Dissolved Gases (N2, CO2, O2)
- Waste
- Plasma carbohydrates → mainly glucose
- Plasma lipids → mainly paired with plasma proteins
- Plasma proteins:
Plasma Protein
Type
Albumin
Amt (g/L)
45
Role
See below
Type
Globulins
α1
Amt (g/L)
25
Role
α-1 antitrypsin – protease inhibitor, inhibits trypsin, chymotrypsin, plasma
Lipoproteins – chylomicrons, VLDL, LDL, HDL → transport lipids
α1 acid glycoprotein – binding basic drugs, acute phase reactant
Type
α2
Amt (g/L)
25
Role
α2 macroglobulin – protease inhibitor
Prothrombin – coagulation
Haptoglobin – scavenges free Hb (intravascular haemolysis)
Caeruloplasmin – binds copper, acute phase reactant
Type
β
Amt (g/L)
25
Role
Transferrin – binds 2Fe3+
Type
γ
Amt (g/L)
25
Role
Immunoglobulins → from B lymphocytes
IgG
76%
Binds complement
IgA
16%
Present in seromucous secretions
IgM
7%
Fixes complement
IgE
1%
Binds MCs in hypersensitivity
Type
Fibrinogen
Amt (g/L)
3
Role
Final step of coagulation → fibrin clot
Type
Cytokine
Amt (g/L)
Role
IL, IFN, chemokines, lymphokines, TNF → mediate immune response
Type
Complement
Amt (g/L)
Role
25 proteins produced by liver/RES → innate immunity amplification
Type
CRP
Amt (g/L)
Role
Inflammatory mediator
Type
Coagulation
Amt (g/L)
Role
Trigger coagulation cascade – biological amplification system
Role of Albumin
Endogenous albumin is produced exclusively by hepatocytes, 9-12g/day.
Osmotic Pressure
- NFP = k[(Pc – Pi) – σ (πc – πi)]
- Capillaries are impermeable to plasma proteins
- ∴exert oncotic P ~25mmHg, holds fluid intravascularly
- Confers 80% of osmotic pressure
Transport
- Binds acidic, basic and neutral drugs
- Domains I and II are responsible for transport of numerous endogenous & exogenous molecules; eg unconjucgated bilirubin, thyroid hormone
Acid-Base Buffer
- 16 Imidazole residues w pK 6.75
- Buffer extracellular pH
Anti-Oxidant
- Neutralizes by-products of metabolism which would otherwise be converted to free radicals
- Conferred by the cysteine residues in position 34 which expose a -SH radical group (thiol) = one of the main extracellular antioxidants
Anticoagulant
- Enhances AT III activity
Protein Store
- 50% of plasma protein
- Often used as a nutritional marker
Metabolism
- Protein broken down → amino acid → E
- Drug Metabolism
Other
- Inflammatory marker
- Author: Krisoula Zahariou