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BLOOD PLASMA

Blood plasma is a pale yellow fluid. It's total volume in a normal adult is around 2.5 to 3 litres.

The table shows th constituents of plasma as percentages of the total volume

Plasma contains clotting agents and on exposure to air it will form a clot. The clear fluid exuded from clotted whole blood and clotted plasma is called serum. The quantities of the constituents of serum are the same as those for plasma minus the small amount of clotting agents, such as fibrinogen, destroyed during the clotting process.

Blood plasma comprises around 20 % of the boy's extracellular fluid and is very similar in composition to interstitial fluid. The main difference being the protein content. Interstial fluid contains much less protein at around 2% by volume.

This is because most of the plasma protein molecules are too large to pass through the capillary walls into the interstitial area. The small amount of protein which does leak through is eventually taken up by the lymph and then ultimately returned to the blood.

Plasma proteins form three major groups and these have various functions.

The three groups are :

• albumin (60% of total plasma protein)
• fibrinogen (4% of total plasma protein)
• globulins (36% of total plasma protein)

further fractions (alpha, beta and gamma) can be distinguished within the globulin group.

The relative proportions of plasma proteins can vary in certain diseases and electrophoretic tracings showing such changes can be a useful diagnostic aid.

Most of the plasma proteins are produced by the liver. The gamma globulins are produced by cells of the body's immune system.

Albumin is the smallest of the plasma proteins and is just small enough to pass through capillary walls. In normal circumstances this leads to the small amount of leakage into the interstitial fluid alredy mentioned. In svere kidney disease large amounts of albumin are able to leak out through the damaged kidney tubules and can be detected in the urine. Because the liver can quickly and easily replace lost albumin the body may lose large amounts of the protein without showing signs of disease.

Functions of the plasma proteins include :

• Intravascular osmotic effect. This is important in maintaining fluid and electrolyte balance and is discussed more fully in those sessions.
• Contributes to the viscosity of the plasma
• Transport of insoluble substances around the body by allowing them to bind to protein molecules.
• Protein reserve for the body
• Clotting this is discussed further in sessions on wounds and wound healing
• Inflammatory response again discussed further in the sessions on the immune system.
• Protection from infection the gamma globulins function as antibodies and their role is discussed more fully in the sessions on the immune system.
• Maintenance of the acid-base balance. Again this is explored in more detail in the session on fluid and electrolyte balance.

The blood plasma contains inorganic ions which are important in regulating cell function and maintaining homeostasis. For example depletion of potassium may occur following severe diarrhoea and vomiting. Potassium affects cell excitability and severe loss will cause muscle weakness and abnormalities of the cardiac impulse. The same problems may result in severe sodium depletion. Lack odf sodium in the plasma will result in a reduction in the overall volume of extracellular fluid which in turn leads to a drop in blood pressure causing weakness, dizziness, mental confusion and fainting.

The above table shows the normal range of concentration of inorganic ions in the blood plasma.

In addition to proteins and inorganic ions the blood plasma carries a wide range of substances in transit to various tissues throughout the body. These include dissolved gases (mainly carbon dioxide). Oxygen is not very soluble in water hence the need for the specialised oxygen transporing red blood cells.

Nutrients are carried in the blood plasma. The most abundant being glucose which is the primary source of energy for cell metabolism. Other nutrients in transit in the plasma include amino acids, fatty acids, triglycerides, cholesterol and vitamins.

Waste products of metabolism are also transported bty the plasma including urea, uric acid and cratinine from the kidneys and bilirubin from the gall bladder.

Hormones, such as cortisol and thyroxine are also transported around the body in plasma attached to plasma proteins.

Other substances can be transported in the plasma the most obvious examples being drugs and alcohol.

This page last updated Wednesday, 28 April 1999 21:04 +0100

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