Write concisely on the functions of plasma proteins.
Outline:
·
What is plasma?
·
Origin of plasma proteins
·
Role in:
- Transport
- Hemostasis
- Clotting
- Defense
·
Clinical consequence of deficiency
Essay:
The fluid portion of the blood, the plasma, is a remarkable solution
containing an immense number of ions, inorganic molecules, and organic molecules
that are in transit to various parts of the body or aid in the transport of
other substances. The normal plasma volume is about 5% of body weight. Plasma
consist of 92% water, 7% proteins and 1 % dissolved organic molecules, ions,
trace elements, oxygen and carbon dioxide.
The plasma proteins consist of albumin, globulin, and fibrinogen
fractions. The albumins are manufactured in the liver; they are major
contributors to the osmotic pressure of plasma and serves as carriers for
different substrates. In normal adult humans, the plasma albumin level is
3.5-5.0 g/dL, and the total exchangeable albumin pool is 4.0-5.0 g/kg body
weight; 38-45% of this albumin is intravascular, and much of the rest of it is
in the skin. Albumin synthesis is carefully regulated. It is decreased during
fasting and increased in conditions such as nephrosis in which there is
excessive albumin loss. The globulins are manufactured in the liver and lymphoid
tissue and they include antibodies, enzymes, and clotting factors. Fibrinogens
are concerned with blood clotting (fibrinogen, prothrombin, etc.) and they are
manufactured in the liver.
Plasma proteins play a crucial role in the transport of various
substances in blood, hemostasis, blood clotting and defense.
Plasma proteins act as a transport of various substances which is
otherwise insoluble in plasma or too low in molecular weight and if left free,
can be filtered by the kidney and excreted from the body. The major protein in
plasma, albumin, transport metal ioins, bile pigments, fatty acids, uric acid
and hormones such as growth hormone and testosterone. Globulins also serve as
carriers for the active vitamin D metabolite, cholecalciferol. Some plasma
proteins are specific for the substance it transports. For example,
thyroxine-binding protein (TBG) binds the thyroid hormones, transcortin
transport steroid hormones and transferrin transport iron. Binding keeps these
hormones form being rapidly filtered through the glomeruli and provides a stable
reservoir of hormones on which the tissues can draw.
Hemostasis means the maintenance of static or constant conditions in the
internal environment. The role of plasma proteins in the regulation of
interstitial fluid compartment volume is due mainly to its inability to move out
of the circulatory system into the interstitial fluid which maintains the blood
volume and extracellular fluid volume, preventing any major fluxes between both
compartments. The capillary walls are relatively impermeable to the proteins in
plasma, and the proteins therefore exert an osmotic force of about 25 mmHg
across the capillary wall that tends to pull water into the blood, resulting in
the retention of water in the blood against the hydrostatic pressures that tend
to force water out of the circulatory system. At the arteriolar end, high
hydrostatic pressure is greater than the osmotic pressure of the plasma proteins
and forces fluid out of the capillaries into the interstitial space. As blood
flows in the capillaries, hydrostatic pressure decreases due to resistance to
flow. At the venous end, hydrostatic pressure is now less than the osmotic
pressure of the plasma proteins. This, together with the hydrostatic pressure of
the tissues, causes fluid to move back into the capillary. The interaction
between hydrostatic and osmotic forces allow exchange of materials to take place
between blood plasma and the interstitial tissue fluid. Nutrients and other
important metabolites enters the interstitial compartment at the arterial end
while waste materials enters the circulatory system at the venous end for
transportation to the kidneys for removal. Plasma protein levels are low during
prolonged starvation, malabsorption syndrome due to intestinal diseases such as
spruce, and liver disease, and because of the decrease in plasma oncotic
pressure, edema tends to develop.
The plasma proteins are also responsible for 15% of the buffering
capacity of the blood because of the weak ionization of their COOH and NH2
groups. This helps to maintain the plasma pH at around 7.40, which is vital for
the optimal functioning of enzymes and membrane transporters.
Plasma proteins like fibrinogens are essential for the process of blood
clotting. When a small blood vessel is transected or damaged, the injury
initiates a series of events that leads to the formation of a clot. This seals
off the damaged region and prevents further blood loss. Blood clotting factors
in plasma are inactive forms of proteolytic enzymes. When converted to the
active forms, their enzymatic actions cause the successive, cascading reactions
of the clotting process. An absence of any of these plasma clotting
proteins/factors will result in defective blood clotting. For example, patients
with hemophilia A can bleed to death from small cuts as their blood lacks Factor
VIII and cannot forms blood clots in the event of a vessel rupture or damage.
Gamma globulins in the plasma, called immunoglobubins are antibodies
which play an important role in the defense of the body against bacteria and
foreign antigens. All immunoglobulins are composed of combinations of light and
heavy polypeptide chains. They are secreted into the general circulation by
plasma cells. There are five general classes of antibodies, IgM, IgG, IgA, IgD
and IgE. Antibodies act mainly in two ways to protein the body against invading
agents: by direct attack on the invader and by activation of the complement
system. The antibodies can inactivate the invading agent by agglutination, in
which multiple large particles with antigens on their surfaces are bound
together into a clump; precipitation, in which the molecular complex of soluble
antigen (such as tetanus toxin) and antibody becomes so large that it is
rendered insoluble and precipitates or lysis, in which some potent antibodies
attack membranes of cellular agents and thereby causing rupture of the cell. The
complement system involves another additional 20 proteins, many of which are
enzyme precursors. Binding of an antigen to an antibody uncovers a reactive site
on the antibody which binds to a protein (usually C1) of the complement system,
setting into motion of cascade of sequential reactions such as opsonization and
phagocytosis, chemotaxis and other inflammatory effects.
Plasma proteins have various functions which are necessary for the
maintenance of hemostasis and metabolism of nutrients and hormones.