Plasma Proteins – ORIGIN OF PLASMA PROTEINS
l. In Embryo: Mesenchymal cells through a process of secretion or dissolution of their substances, form plasma proteins. First the albumin is synthesized and rest of plasma proteins afterwards.
2. In Adults
(i) Albumin from liver mainly
(ii) Fibrinogen also from the liver
(iii) Globulin from
(a) Tissue macrophages (i.e. reticulo-endothelial cells,) – Liver (specially synthesize a and globulin), spleen and bone marrow.
(b) Plasma cells – These are large, oval and very active cells found in medullary cords of lymphoid follicles and small lymphocytes.
(c) Lymphocytes synthesize y-globulin
FORMS OF PLASMA PROTEINS AND THEIR FUNCTIONS
Normal total plasma protein concentration: 6.4-8.3 gm/ dL of blood.
Serum proteins means all plasma proteins minus fibrinogen.
Forms of plasma proteins and their functions
The major forms of plasma proteins and their functions
are summarized in
ALBUMIN
GLOBULIN
Normal plasma concentration: 2-3 gm/ dL
(Average: 2.3 gm/ dL).
Forms of globulin
Types Normal level
(i) 13% a-globulin (<Xi; «xi) 0.78–0.81 gm/ dL
(ii) 14% P-globulin (P1; P2) 0.79–0.84 gm/dL
(iii) 11 % y-globulin (y1; ‘Y2) 0.66–0.70 gm/dL
Physiology MCQs Plasma Proteins MPT Preparation Quiz Mode -Click
FUNCTIONS OF PLASMA PROTEINS
1. Helps in coagulation of blood due to presence of fibrinogen, prothrombin and other coagulation factors which are protein in nature.
2. Helps to maintain colloidal osmotic pressure: (COP) across the capillary wall; normally it is 25-30 mmHg. How?
(i) Osmotic pressure across the capillary wall can be exerted both by
(a) the crystalloids e.g. urea,. Na+, glucose etc., and
(b) the colloids e.g. plasma1 proteins. However, capillary wall is completely permeable to crystalloids, therefore, crystalloids hardly contribute to capillary osmotic pressure and proteins exer1r an osmotic force of 25 mmHg across the capillary wall.
(ii) COP is inversely proportional to the molecular size and shape, and is directly related to the
concentration of molecules. Therefore, 80% of COP is due to albumin because of least molecular weight (i.e. molecule size) and maximum concentration.
(iii) COP across the capillary wall helps to maintain
the exchange of fluid at tissue level. The rate of
fluid exchange (i.e. filtration-absorption) at any point
along a capillary depends upon a balance of forces,
called starling force
COP due to the plasma colloids is called the oncotic
pressure
(a) Hydrostatic pressure across capillary wall – it favours filtration.
(b) COP across capillary wall – it favours absorption.
(c) Hydrostatic pressure in interstitial fluid: Normal 2-3 mm.Hg. {
(d) Interstitial fluid osmotic pressure: Normal 3-4 mmHg.
(c) and (d) essentially cancel each other, therefore, forces which determine fluid exchange at tissue level are ·
(a) and (b). Hydrostatic pressure at arteriolar end is 37 mmHg, therefore, some fluid is forced out of capillary bed: 37 – 25 = 12 mmHg. Hydrostatic pressure at venous end is 15 mmHg, therefore, some fluid will be pulled back by osmotic forces: 25 – 15 = 10 mmHg.
Physiology MCQs Plasma Proteins MPT Preparation Quiz Mode -Click
Applied
(a) Hypoproteinaemia (i.e. decrease in plasma protein level) causes decrease in COP, therefore, increase filtration occurs at arterial end and decrease in absorption of fluid at venous end, resulting in abnormal collection of fluid in interstitial spaces, called OEDEMA.
(b) When capillary permeability is increased e.g. in anoxia, urticaria, inflammation etc., all the proteins escape much more readily from the capillary into interstitial spaces producing oederna.
3. Helps in maintaining viscosity of blood. How? The viscosity of a protein depends on:
(i) the shape of the protein molecules (mainly), and
(ii) the size of the protein molecules. The less symmetrical the molecule (like fibrinogen), the greater is its viscosity. Since 80% of total plasma protein concentration is due to albumin, and fibrinogen is present in traces, blood viscosity is maintained at low level. Normally viscosity of blood is 4-5 times that of water.
4. Helps in maintaining systemic arterial blood pressure constant. The resistance to flow of fluid at constant velocity through a capillary depends almost entirely on viscosity of fluid. The arterial blood pressure i.e. resistance to blood flow is directly proportional to viscosity of blood. However, plasma proteins maintain the blood pressure constant by maintaining viscosity of blood.
5. Provides stability to blood due to presence of globulin and fibrinogen. If blood loses its stability, it will lead to Rouleaux formation of RBCs i.e. RBCs pile one over another.
6. Helps in maintaining the acid-base balance in the body. Plasma proteins act as buffers. Their buffering capacity is 1/6th (about 15%-16%) of total buffering capacity of blood. They are amplwteric in nature i.e. behave as acids or bases depending on the conditions and thereby maintain the blood pH at 7.4, by accepting or donating H+. \ft cl11111ism At physiological blood pH of 7.4, plasma proteins exist in an ‘ionised’ form i.e. (i) ‘C’ terminal end is in the form of ‘COO-, and (ii) ‘N’ terminal end is in the form of -NH3 +. Therefore, ‘C’ terminal end can buffer the change that may follow addition of an acid whereas ‘N’ terminal end can buffer the change that may follow addition of an alkali
7. Leucocytes can manufacture a few substances called Trepfw,ies or Carrel from plasma proteins, which help in the nutrition of tissues.
8. Immune function. y-globulin produce antibodies which provide immunity to the body.
9. Transport function. Plasma proteins combine loosPly with many agents e.g. (i) hormones (thyroxine, cortisol) (ii) drugs (iii) metals (a) ‘transferrin’ ((½ ~1 globulin) with two atoms • of Fe3+, is carried to the site of storage. (b) ‘ceruloplasmin’ ((½ ~1 globulin) with copp1er, circulates in blood.
10. Reservoir function. Plasma proteins form loose bond with the hormones, drugs and metals etc. to serve as a ‘reservoir’, from which the same are released slowly. This plays a beneficial role during starvation.