Plasma

PLASMA, the liquid part of blood, constituting about 50-57% of the total volume of blood. Plasma is a transparent yellowish-green liquid (see vol. XIV, pp. 659-660, figure 16) of slightly alkaline, almost neutral reaction (pH=7.3-7.4), with specific weight 1.028-1.032. Outside the organism, plasma clots within several minutes; after separation of the gelatinous clot containing fibrin, a transparent yellowish-green liquid remains that is incapable of further clotting—blood serum. The composition of P. and its constancy are the result of very complex regulatory processes of metabolism in the organism. Native plasma is obtained by centrifuging fresh blood under conditions of strong cooling or in paraffinized vessels (test tubes, capillaries). Usually, however, P. obtained by centrifuging blood with the addition of substances that prevent clotting is used. Such so-called stabilizers include hirudin, heparin, Bayer 205, novirudin, and oxalic, citric, and fluorine salts of sodium. The best stabilizer is hirudin, a few crystals of which (for 5-10 cm³ of blood) are sufficient to delay clotting for several hours. Heparin also delays clotting very well. Among the salts, sodium oxalate is the most active (20 mg prevents clotting of 50-100 cm³ of blood), but when it is used, as well as when sodium fluoride is used, a precipitate of calcium salts often forms, which is a disadvantage of these salts compared to sodium citrate. The latter leaves P. without noticeable changes. The addition of novirudin to P. gives it a dark chocolate tint, although it apparently has little effect on its physicochemical properties. P. contains 90% water and 10% solid ingredients, some of which are in P. in a colloidal and some in a crystalloid solution. The water content in normal P. is quite constant and shows only small fluctuations with abundant or insufficient introduction of fluids, with various losses of fluid by the body, profuse sweating, etc. (for more details, see Water metabolism, Hydremia).—For the components of P.—see Blood, chemical composition of blood.