|MadSci Network: Medicine|
Plasma is the liquid portion of the blood that is separated from the blood cells by centrifugation. One of the characteristics of plasma is that it clots easily which is important for hemophiliacs needing a transfusion but is a nuisance in most other applications. By agitating the plasma, one can precipitate the clotting factors as a large clot, and the leftover fluid is called serum. So, serum plus clotting factors is plasma, and clotted plasma yields serum (as an interesting aside, "serum" is Latin for whey, the liquid portion of clotted milk removed in making cheese).
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