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4 Albumin Albumin makes up more than one-half of the total plasma proteins and has a molecular weight of 69,000 daltons. Because of its relatively small molecular size and its high concentration, albumin contributes to 75 – 80% of the colloid osmotic pres- sure of plasma. Therefore, it plays a major role in the regulation of the intravascular volume and the fluid exchange between the vascular system and the extravascular space. In addition, albumin serves as a transport protein for various substances, including small ions (for example, calcium and iodine) and organic compounds (for example, bilirubin). Immunoglobulins The immunoglobulins, which represent approximately one-sixth of the total protein content, largely constitute the gamma-globulin fraction. The immunoglobulins are antibodies circulating in the blood and thus are also called humoral anti- bodies. They are of great importance in an organism's defense against infectious agents and other foreign substances. Of the different classes of immunoglobulins that can be distinguished, the principal ones are IgG, IgM, IgA, IgD, and IgE. Antibodies are formed under the stimulus of a foreign substance, the antigen, and are specifically directed against this antigen. The specific- ity of an antibody reaction appears to be ensured by a specific molecular structure in certain parts of the antibody molecule. Other proteins The lipoproteins comprise another class of serum proteins and make up slightly less than 10% of the total. Presumably, they serve as transport proteins for lipids, including cholesterol, cholesterol esters, and phospholipids. Other serum proteins that function as carriers for specific substances are transport proteins for metal ions (for example, the iron-binding protein, transferrin, and the copper-binding protein, ceruloplasmin). Several serum proteins have the capacity to inhibit proteolytic enzymes, thereby protecting serum and possibly tissue pro- teins against enzymatic degradation. Also present in apprecia- ble concentrations are several other glycoproteins, numerous enzymes, and an array of clotting factors. The complement components constitute another key group. Complement is an important effector system in immune reac- tions, with the target being the cell surface membranes. Its action may result in cell lysis, directed migration of cells, and preparation of damaged cells for uptake and removal by other cells (via phagocytosis). Other constituents In addition to the proteins, many other important classes of compounds circulate in the blood plasma. Most of these are smaller molecules that diffuse freely through cell membranes; therefore, they are more similarly distributed throughout all the fluids of the body and are not as characteristic for plasma or serum as the proteins. In terms of their concentration and their function, the electrolytes are most important. They are the primary factors in the regulation of the osmotic pressure of plasma, and they contribute also to pH control. The chief cations are sodium, potassium, calcium, and magnesium. The chief anions are chloride, bicarbonate, phosphate, sulfate, and organic acids. Glucose is also an important constituent of plasma because it is a major source of energy for cells throughout the body. In addition, free amino acids, the constituents of peptides and proteins, circulate in plasma. Blood coagulation When mammalian blood is shed, it congeals rapidly into a gelatinous clot of enmeshed fibrin threads that trap blood cells and serum (Fig. 5). Modern hematology indicates a succession of reactions leading to the formation of insoluble fibrin from a soluble precursor, fibrinogen (factor I). Thrombin The agent ultimately responsible for the formation of fibrin is a proteolytic enzyme, thrombin, which splits four small poly- peptide fragments (the fibrinopeptides) from each molecule of fibrinogen. The remainder of the fibrinogen molecule, now called the fibrin monomer, polymerizes to form insoluble fibrin, that is, the structure of the clot. The strands of fibrin are given added strength through covalent bonds between adjacent Blood (continued) + ward ' s science Fig. 4: Colorized light micrograph of bone marrow indicating the megakaryocytes (colorized blue), which form blood platelets. Dark purple cells are the erythroplastic islets, which form the red blood cells (erythrocytes). Pale purple cells are granulocyte islets, which give rise to the white blood cells (leukocytes). Red reticular supporting cells are also shown. (Credit: Science Photo Library/Alamy Stock Photo)