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The fluid portion of the blood, mainly composed of water (90%) but also contains a range of proteins(6.5-8%) and low molecular weight substances(2%). When the blood is separated using centrifugation plasma is viewed as a pale yellow liquid and forms 55% of the total volume of the blood. Plasma holds a key role in the maintaining of osmotic pressure and acts as a transport medium for a range of molecules. Serum is plasma after the removal of clotting factors II, V, VIII and fibrinogen.

Plasma Proteins
Over 100 different types of plasma protein exist but they can be grouped into 3 fractions; Plasma Albumin, Plasma Globulin and Fibrinogen.

Alternatively it is possible to group plasma proteins according to their function;

Low Molecular Weight substances
These substances include K+, Na+, Ca2+, HCO3-, glucose and urea. These molecules and ions all produce osmotic pressure, also in the case of HCO3- forms the principle buffering system of the blood. The combined effect of these molecules results in plasma having an osmolarity of 280-290 mOsm/L. This osmolarity is equal to that of 0.9 % saline solution hence its use to hydrate patients intravenously.

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