Serum protein is composed of albumin and globulin, either nonbound or acting as carrier proteins. The word “globulin” is an old chemical fractionation term that refers to the non-albumin portion of serum protein; it was subsequently found that this portion includes a heterologous group of proteins such as glycoproteins, lipoproteins, and immunoglobulins. Most globulin molecules are considerably larger than albumin, although the total quantity of albumin is normally two to three times the level of globulin. Albumin seems most active in maintaining the serum oncotic pressure, where it normally has about 4 times as much importance as globulin, accounting for about 80% of the plasma oncotic pressure. Albumin also acts as a transport protein for some drugs and a few other substances. The globulins have more varied assignments than albumin and form the main transport system for various substances as well as constituting the antibody system, the clotting proteins, complement, and certain special-duty substances such as the “acute reaction” proteins. Most serum albumin is produced by the liver. Some of the globulins are produced by the liver, some by the reticuloendothelial system, and some by other tissues or by poorly understood mechanisms.
Plasma contains fibrinogen in addition to the ordinary serum proteins.