Albumin is the main protein constituent of the plasma, accounting for approximately half of the total plasma proteins. It is the smallest plasma protein (about 70 kilodaltons) and is made in the liver. Albumin is responsible for exerting the concentration gradient between blood and extracellular tissue fluid. This major osmotic pressure on the blood vessel wall, called the colloid osmotic pressure, maintains the correct proportion of blood to tissue fluid volume. If a significant amount of albumin leaks out of the blood vessels into the loose connective tissue or is lost from the blood to urine in the kidneys, then the colloid osmotic pressure of the blood decreases, and ﬂuid accumulates in the tissues. (This increase in tissue ﬂuid is most
TABLE Composition of Blood Plasma10.2 Component % Water 91–92 Protein (albumin, globulins, fibrinogen) 7–8 Other solutes: 1–2 • Electrolytes (Na, K, Ca2, Mg2, Cl, HCO3 , PO4 3, SO4 2) • Nonprotein nitrogen substances (urea, uric acid, creatine, creatinine, ammonium salts) • Nutrients (glucose, lipids, amino acids) • Blood gases (oxygen, carbon dioxide, nitrogen) • Regulatory substances (hormones, enzymes) readily noted by swelling of the ankles at the end of a day.)
Albumin also acts as a carrier protein; it binds and transports hormones (thyroxine), metabolites (bilirubin), and drugs (barbiturates).
Globulins include the immunoglobulins (-globulins), the largest component of the globulin fraction, and nonimmune globulins (-globulin and -globulin). The immunoglobulins are antibodies, a class of functional immune-system molecules secreted by plasma cells. (Antibodies are discussed in Chapter 14, Lymphatic System.)
Nonimmune globulins are secreted by the liver. They help maintain the osmotic pressure within the vascular system and also serve as carrier proteins for various substances such as copper (by ceruloplasmin), iron (by transferrin), and the protein hemoglobin (by haptoglobin). Nonimmune globulins also include fibronectin, lipoproteins, coagulation factors, and other molecules that may exchange between the blood and the extravascular connective tissue.
Fibrinogen, the largest plasma protein (340 kilodaltons), is made in the liver. In a series of cascade reactions with other coagulation factors, soluble fibrinogen is transformed into the insoluble protein fibrin (323 kilodaltons). During conversion of fibrinogen to fibrin, fibrinogen chains are broken to produce fibrin monomers that rapidly polymerize to form long fibers. These fibers become cross-linked to form an impermeable net at the site of damaged blood vessels, thereby preventing further blood loss.
With the exception of these large plasma proteins and regulatory substances, which are small proteins or polypeptides, most plasma constituents are small enough to pass through the blood vessel wall into the extracellular spaces of the adjacent connective tissue.
In general, plasma proteins react with common fixatives; they are often retained within the blood vessels in tissue sections. Plasma proteins do not possess a structural form above the molecular level; thus, when they are retained in blood vessels in the tissue block, they appear as a homogeneous substance that stains evenly with eosin in hematoxylin and eosin (H&E)–stained sections.
Serum is the same as blood plasma except that clotting factors have been removed.