PLASMA AND PLASMA PROTEINS
After cellular elements are removed from blood, the remaining liquid portion is called plasma. More than 90% of plasma is water. The remainder consists primarily of plasma proteins, clot-ting factors (particularly fibrinogen), and small amounts of other substances such as nutrients, enzymes, waste products, and gases. If plasma is allowed to clot, the remaining fluid is called serum. Serum has essentially the same composition as plasma, except that fibrinogen and several clotting factors have been removed in the clotting process.
Plasma proteins consist primarily of albumin and globulins. The globulins can be separated into three main fractions—alpha, beta, and gamma—each of which consists of distinct proteins that have different functions. Important proteins in the alpha and beta fractions are the transport globulins and the clotting factors that are made in the liver. The transport globulins carry various substances in bound form around the circulation. For example, thyroid-binding globulin carries thyroxin, and transferrin carries iron. The clotting factors, including fibrinogen, remain in an inac-tive form in the blood plasma until activated by the clotting cas-cade. The gamma globulin fraction refers to the immunoglobulins, or antibodies. These proteins are produced by the well-differentiated lymphocytes and plasma cells. The actual fractionation of the globulins can be seen on a specific laboratory test (serum protein electrophoresis).
Albumin is particularly important for the maintenance of fluid balance within the vascular system. Capillary walls are imperme-able to albumin, so its presence in the plasma creates an osmotic force that keeps fluid within the vascular space. Albumin, which is produced by the liver, has the capacity to bind to several sub-stances that are transported in plasma (eg, certain medications, bilirubin, some hormones). People with poor hepatic function may have low concentrations of albumin, with a resultant de-crease in osmotic pressure and the development of edema.