Blood plasma
Blood plasma is the liquid component of
blood, in which the
blood cells are suspended. Plasma is the largest single component of blood comprising about 55% of total blood volume. The term
Serum refers to blood plasma in which
clotting factors (such as
fibrin) have been removed. Blood plasma contains many vital proteins including
fibrinogen,
globulins and
human serum albumin.
Plasma resembles
whey in appearance (transparent with a faint straw colour). It is mainly composed of
water,
blood proteins, and
inorganic electrolytes. It serves as transport medium for
glucose,
lipids,
amino acids,
hormones,
metabolic end products,
carbon dioxide and
oxygen. (Oxygen transport capacity of plasma is much lower than that of the
hemoglobin in the
red blood cells; it may increase under
hyperbaric conditions.) Plasma is the storage and transport medium of clotting factors and its protein content is necessary to maintain the
oncotic pressure of the blood.
For
hematalogical,
biochemical,
blood bank-related,
immunological and other kinds of tests, plasma is obtained from whole blood. To prevent
clotting, an
anticoagulant such as
citrate or
heparin is added to the blood specimen immediately after it is obtained. (Usually the anticoagulant is already in the evacuated blood collection tube (e.g.
Vacutainer or Vacuette®) when the patient is
bled.) The specimen is then
centrifuged to separate plasma from blood cells. Plasma can be frozen below -20
°C nearly indefinitely for subsequent analysis or use. This blood product derivative is known as
fresh frozen plasma (FFP).
For many
biochemical laboratory tests, plasma and blood serum can be used interchangeably. Serum resembles plasma in composition but lacks the coagulation factors. It is obtained by letting a blood specimen clot prior to centrifugation. For this purpose, a
serum-separating tube (SST) can be used which contains an inert
catalyst (such as glass beads or powder) to facilitate clotting as well as a portion of gel with a density designed to sit between the liquid and cellular layers in the tube after centrifugation, making separation more convenient.
Tests of
coagulation (such as the
INR and
aPTT) require all clotting factors to be preserved. Serum, therefore, is inappropriate for these tests. A citrated evacuated blood collection tube (e.g.
Vacutainer, Vacutube or Vacuette) is usually used, as the anticoagulant effects of citrate are dependent upon concentration and can be reversed for testing.
Serum is preferred for many tests as the anticoagulants in plasma can sometimes interfere with the results. Different anticoagulants interfere with different tests; using serum means the same sample can be used for many tests. In
protein electrophoresis, using plasma causes an additional band to be seen, which might be mistaken for a
paraprotein.
Serum is also commonly used as a supplement added to a particular cell culture media to ensure a successful growth of the cells. This is because the serum provides the cells with numerous proteins, nutrients, hormones and attachment factors that are beneficial. [
1] One of the more common serum added is the foetal calf serum (FCS).
Fresh frozen plasma (FFP) is prepared from a single unit of blood. It is frozen after collection and can be stored for one year from date of collection. FFP contains all of the coagulation factors and proteins present in the original unit of blood. It is used to treat
coagulopathies from
warfarin overdose,
liver disease, or dilutional coagulopathy. FFP that has been stored more than a standard length of time is re-classified as simply "frozen plasma", which is identical except that the coagulation factors are no longer considered completely viable.
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Dried plasma packages used by Britain and US military during WWII |
Dried plasma was developed and first used during
WWII. Prior to the United States involvement in the war, liquid plasma and whole blood were used. The "Blood for Britain" program during the early 1940s was quite successful (and popular stateside). Nonetheless the decision was made to develop a dried plasma package for the armed forces because it reduced breakage and made transport, packaging, and storage much simpler. [
2]
The resulting Army-Navy dried plasma package came in two tin cans containing 400 cc bottles. One bottle contained enough distilled water to completely reconstitute the dried plasma contained in the other bottle. In about three minutes, the plasma would be ready to use and could stay fresh for around four hours. [
3]
By the end of the war the American Red Cross had provided enough blood for over six million plasma packages. Most of the surplus plasma was returned stateside for civilian use.
Serum albumin replaced dried plasma for combat use during the
Korean War. [
4]
