Proteins, also called polypeptides, consist of amino acids linked by peptide bonds. Although all human proteins are constructed from a mere 20 amino acids, variations in chain length, amino acid sequence, and incorporated constituents combine to make possible an almost infinite number of protein molecules. All cells manufacture proteins, with different proteins characterizing different cell types. The amino acids needed for these processes enter the body from dietary sources. These amino acids are rapidly distributed to tissue cells, which promptly incorporate them into proteins.
Three-fourths of the body's solid matter is protein and, except for hemoglobin, relatively little circulates in whole blood. The major plasma proteins are albumin, the globulins, and fibrinogen. Fibrinogen evolves into insoluble fibrin when blood coagulates. The fluid that remains after coagulation is called serum. Serum and plasma have the same protein composition except that serum lacks fibrinogen and several other coagulation factors (prothrombin, factor VIII, factor V, and factor XIII).
The proteins in circulating blood transport amino acids from one site to another, providing raw materials for synthesis, degradation, and metabolic interconversion. Circulating proteins also function as buffers in acid-base balance, contribute to the maintenance of colloidal osmotic pressure, and aid in transporting lipids, enzymes, hormones, vitamins, and certain minerals.
Most plasma proteins originate in the liver. Hepatocytes synthesize fibrinogen, albumin, and 60 to 80 percent of the globulins. The remaining globulins are immunoglobulins (antibodies), which are manufactured by the lymphoreticular system. Immunoglobulins are studied as part of the immune system (see Chapter 3 - Immunology and Immunologic Testing), whereas fibrinogen is usually studied as part of a coagulation workup (see Chapter 2 - Hemostasis and Tests of Hemostatic Functions). The focus of this section is on the major serum proteins (albumin and nonantibody globulins), binding proteins, and protein metabolites.17