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Lesson 8-2 Coagulation System

COAGULATION FACTORS

Table 2-1 contains a complete list of coagulation factors and their synonyms.

Table 2-1. Nomenclature of coagulation factors.

Factor I (Fibrinogen). Fibrinogen, a plasma glycoprotein, is converted into fibrin in the presence of thrombin. The major source of fibrinogen is the liver. A minimum of 50 to 100 mg/dL is required for normal coagulation. Bleeding due to a fibrinogen deficiency does not usually become manifest until the plasma concentration is below 75 mg per dl. Decreased levels of fibrinogen can be caused by several reasons -- decreased liver production is due to acute hepatitis or cirrosis; fibrinolysins, which attack both fibrin and fibrinogen molecules; and massive production of fibrin, as seen in disseminated intravascular coagulation (DIC). Replenishment can be achieved by administration of fresh frozen plasma or cryoprecipitates.

Factor II (Prothrombin). This substance is a stable glycoprotein, synthesized in the liver if an adequate amount of vitamin K is available. Prothrombin (a proenzyme) is the inactive precursor of thrombin. When Vitamin K is absent, various types of prothrombin molecules still form, but the site on the molecule that would react in the coagulation scheme is left inactive and nonfunctional. Factors II, VII, IX, and X are part of the prothrombin complex; all are vitamin K dependent, meaning that these factors cannot be produced without the presence of vitamin K in the liver in adequate amounts. These factors are absorbed by both BaSO4 (barium sulfate) and Al(OH)3 (aluminum hydroxide). With the exception of Factor II, all are found in serum.

Factor III (Tissue Thromboplastin). Thromboplastin, or tissue factor is a high-molecular-weight lipoprotein found in most of the body tissues with increased concentrations in the lungs and brain. It is probably the phospholipid content of tissue thromboplastin that makes platelets unnecessary in stage I of the coagulation process. Tissue thromboplastin requires calcium and factors V, VII, and X to convert prothrombin to thrombin. It is found in the brain, lung, vascular endothelium, liver, placenta, or kidneys.

Factor IV ( Ionized Calcium). Calcium is an inorganic ion that is necessary for clotting to occur. The exact mechanism by which calcium acts in the coagulation process is not known. The fact that it is essential for coagulation makes possible the use of anticoagulants, which merely bind up the calcium and, therefore, completely inhibit coagulation. It is unlikely, however, that a bleeding tendency is ever caused by a deficiency of calcium, since clinical tetany occurs with higher levels of calcium than are necessary for coagulation.

Factor V (Labile Factor Proaccelerin, Accelerator Globulin). Factor V is derived from plasma globulin, and it acts as an accelerator in the conversion of prothrombin to thrombin in the presence of tissue thromboplastin. Factor V is not present in serum because it is consumed during the clotting of blood. In addition, it rapidly is inactivated during storage. It is not absorbed by BaSO4 (barium sulfate).

Factor VI (Accelerin). Factor VI has been eliminated as an entity by the International Committee on Blood Clotting Factors.

Factor VII (Stable Factor, Proconvertin). Factor VII is stable to both heat and storage. It is thought to act as an accelerator in the conversion of prothrombin to thrombin. Factor VII is not consumed in the clotting process; therefore, it has a high concentration in serum and plasma. Factor VII activity may actually increase the coagulation process. A vitamin K dependent factor is manufactured in the liver.

Factor VIII (Antihemophilic Factor, Antihemophilic Globulin). Factor VIII is essential to the formation of intrinsic blood thromboplastin in the first stage of clotting. Factor VIII is a combination of two subunits: VIII:C and VIII:vWF( von Willebrandís factor). Deficiency of factor VIII results in the reduction of thromboplastin as well as decreased conversion of prothrombin. Factor VIII deficiency is a hereditary sex-linked disorder, which is transmitted by females and manifested almost exclusively in males (hemophilia A). The unit is measured by the activated partial thromboplastin time (APTT) test.

Factor IX (Plasma Thromboplastin Component, Christmas Factor). Factor IX influences the amount of thromboplastin formed. This factor is not consumed in the clotting process; therefore, it is present in serum. Deficiency of factor IX is either hereditary or acquired and is known as hemophilia B or Christmas disease.

Factor X (Stuart Prower Factor). It is found in both serum and plasma. Factor X aids in the prompt conversion of prothrombin to thrombin. Deficiency of factor X is either acquired or hereditary.

Factor XI (Plasma Thromboplastin Antecedent). Factor XI aids in the formation of plasma thromboplastin. This factor is stable and is found in plasma or serum. It is synthesized in the liver and vitamin K is not required for production. Deficiency of factor XI is probably hereditary and results in a mild hemophilia.

Factor XII (Hageman Factor, Glass Factor). This factor is not required for normal hemostasis, but it is important in the various in vitro assays of the clotting mechanisms. It is a plasma contact factor with glass and is absorbed onto glass. Factor XII is related to factor XI in the activation of thromboplastin, and behaves like an enzyme for which one substrate is factor XI.

Factor XIII (Fibrin Stabilizing Factor, Fibrinase). Factor XIII converts a loosely linked, fibrin clot (in the presence of the calcium ions) into a tough gel. Its activity is greatly reduced in serum (as compared with plasma) because of its strong adsorption of fibrin.


PLATELET FACTORS

Platelets are active in blood coagulation. They perform the following functions: aid in vasoconstriction and the formation of a hemostatic plug, thromboplastic activity, and clot retraction. When platelets contact a wettable surface, at first they adhere to one another and then rupture, releasing chemical factors.

Platelet Factor 1. This factor accelerates prothrombin conversion and is actually blood factor V adsorbed on platelets. (No longer used conventionally.)

Platelet Factor 2. Factor 2 accelerates fibrinogen clotting of thrombin. (No longer used conventionally.)

Platelet Factor 3. A phospholipid substance, found in the platelet membrane, is involved in prothrombin activation. This is the most important factor and probably is an actual intrinsic component of platelets.

Platelet Factor 4. This factor reacts to neutralize heparin.

Platelet Factor 5. This factor is an adsorbed intrinsic fibrinogen. (No longer used conventionally.)

Platelet Factor 6. This factor reduces fibrinolytic activity. (No longer used conventionally.)

Platelet Factor 7. This factor is adsorbed blood factor VII. (No longer used conventionally.)


FIBRINOLYTIC FACTORS

Fibrinolysis is the dissolution of a fibrin clot. The process is a necessary activity following clot formation. The mechanism of clot dissolution is complex and involves a variety of factors. In active circulating plasma profibrinolysin (plasminogen) is converted to its active form, fibrinolysin (plasmin), by tissue activators, streptokinase, urokinase, and other unknown activators. Fibrinolysin acts locally to dissolve the clot.

 

David L. Heiserman, Editor

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Revised: June 06, 2015