When blood is removed from the body and placed in a glass test tube, it clots fairly quickly (Table 1). Calcium ions are required for this process. Addition of EDTA or citrate prevents clotting by binding calcium. Clotting can be initiated in vitro at a later time by adding back an excess of calcium ions. As shown in Table 1, recalcified plasma (minus platelets and other blood cells) will clot in 2-4 min. The clotting time after recalcification can be shortened by adding an emulsion of negatively-charged phospholipids (PL). The clotting time is further shortened to 21-32 sec by preincubation of the plasma with particulate substances such as kaolin (insoluble aluminum silicate). The reaction initiated by kaolin, PL, and calcium is termed the activated partial thromboplastin time (aPTT) test. Alternatively, the clotting time of recalcified plasma can be shortened to 11-12 sec by adding "thromboplastin" (a saline brain extract containing tissue factor, a lipoprotein described below). The reaction initiated by thromboplastin and calcium is termed the prothrombin time (PT) test.
A. "Intrinsic" and "extrinsic" coagulation pathways. Many patients with inherited bleeding disorders have prolongation of the aPTT, the PT, or both. Thus, two pathways for coagulation were proposed (Fig. 1). A patient with a prolonged aPTT and a normal PT is considered to have a defect in the "intrinsic" coagulation pathway. The name indicates that all of the components of the aPTT test, except kaolin, are "intrinsic" to the plasma. On the other hand, a patient with a prolonged PT and a normal aPTT has a defect in the "extrinsic" coagulation pathway (tissue factor is "extrinsic" to the plasma). Prolongation of both the aPTT and the PT suggests that the defect lies in a common pathway.
B. Identification of distinct coagulation factors. Early investigators discovered that mixing plasma from two patients with defects in the same coagulation pathway would sometimes correct the clotting time to normal. This type of result suggests that the two patients have different coagulation factor deficiencies (e.g., the hypothetical orange and blue factors in Fig. 2). In this manner, 11 plasma coagulation factors were discovered: 6 in the "intrinsic" pathway (factors VIII, IX, XI, XII, prekallikrein, and high-molecular weight kininogen), 1 in the "extrinsic" pathway (factor VII), and 4 in the common pathway (factors II, V, X, and fibrinogen).
C. Sequence of coagulation reactions.The classical model of blood coagulation involves a series (or "cascade") of zymogen activation reactions as shown in Fig. 3. At each stage a precursor protein (zymogen) is converted to an active protease by cleavage of one or more peptide bonds in the precursor molecule. The types of components that can be involved at each stage include the following:
(a) a protease (from the preceding stage)
(b) a zymogen
(c) a non-enzymatic protein cofactor
(d) calcium ions
(e) an organizing surface (provided by a phospholipid emulsion in vitro or by platelets in vivo)
The final protease generated is thrombin (factor IIa). Thrombin converts the soluble protein fibrinogen into an insoluble fibrin gel, which is strengthened further by covalent cross-linking catalyzed by factor XIIIa.
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II. Coagulation in vivo
Although the concept of "intrinsic" and "extrinsic" pathways served for many years as a useful model for coagulation, more recent evidence has shown that the pathways are not, in fact, redundant but are highly interconnected. For example, the tissue factor/VIIa complex activates not only factor X (as shown in Fig. 3) but also factor IX of the intrinsic pathway. Furthermore, patients with severe factor VII deficiency may bleed even though the intrinsic pathway is intact. Likewise, the severe bleeding associated with deficiencies of factors VIII or IX would not be expected if the extrinsic pathway alone were sufficient to achieve normal hemostasis.
A. Exposure of plasma to tissue factor initiates coagulation. Tissue factor is a non-enzymatic lipoprotein constitutively expressed on the surface of cells that are not normally in contact with plasma (e.g., fibroblasts and macrophages). Exposure of plasma to these cells initiates coagulation outside a broken blood vessel. Endothelial cells also express tissue factor when stimulated by endotoxin, tumor necrosis factor, or interleukin-1, and may be involved in thrombus formation under pathologic conditions.
Tissue factor binds factor VIIa and accelerates factor X activation about 30,000-fold. Although factor VII is activated by its product, factor Xa, a trace amount of factor VIIa appears to be available in plasma at all times to interact with tissue factor. Factor VIIa also activates factor IX in the presence of tissue factor, providing a connection between the "extrinsic" and "intrinsic" pathways (cf. Figs. 3 & 4). Factors IXa and Xa assemble with their non-enzymatic protein cofactors (VIIIa and Va, respectively) on the surface of aggregated platelets. This leads to local generation of large amounts of Xa and thrombin (IIa), followed by conversion of fibrinogen to fibrin.
Initiation of Coagulation In Vivo
Tissue factor pathway inhibitor (TFPI) is a 34-kDa protein associated with plasma lipoproteins and with the vascular endothelium. It binds to and inhibits factor Xa. The Xa-TFPI complex then interacts with VIIa/tissue factor and inhibits activation of factors X and IX. TFPI may prevent coagulation unless the VIIa/tissue factor initially present generates a sufficient amount of factor IXa to sustain factor X activation via the "intrinsic" pathway. Thus, VIIa/tissue factor may provide the initial stimulus to clot (in the form of relatively small amounts of IXa and Xa) and then be rapidly turned off, while IXa and VIIIa may be responsible for generating the larger amounts of Xa and thrombin required for clot formation.
B. Coagulation can be initiated via the "intrinsic" pathway in vitro when factor XII, prekallikrein, and high-molecular weight kininogen (HMWK) bind to kaolin, glass, or another artificial surface. Once bound, reciprocal activation of XII and prekallikrein occurs (Fig. 3). Factor XIIa triggers clotting via the sequential activation of factors XI, IX, X, and II (prothrombin).
Activation of factor XII is not required for hemostasis, since patients with deficiency of factor XII, prekallikrein, or HMWK do not bleed even though their aPTT values are prolonged.Patients with factor XI deficiency tend to have a mild bleeding disorder, however, implying that XI is involved in hemostasis. The mechanism for activation of factor XI in vivo is unknown, although thrombin has been shown to activate XI in vitro.
C. Concentrations of coagulation factors required for normal hemostasis are summarized in Table 2. With the exception of fibrinogen, factor levels are usually reported as percentages of the concentrations present in plasma pooled from normal individuals. Tissue factor is not present in plasma and cannot be quantified in patients.
Plasma Coagulation Factors
Plasma Concentration (µg/ml)
Required for Hemostasis (% of normal concentration)
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III. Biochemistry of coagulation
A. Structure of coagulation protease zymogens. The structures of the protease zymogens involved in coagulation (i.e., factors II (prothrombin), VII, IX, X, XI, XII, and kallikrein) are shown in Fig. 5. Each protein is secreted by hepatocytes into the bloodstream and contains a signal peptide that is removed during transit into the endoplasmic reticulum. About 200 amino acid residues at the C-terminal end of each zymogen are homologous to trypsin and contain the active site Ser, Asp, and His residues of the protease (catalytic domain). The proteins also contain a variety of domains that are homologous to portions of other proteins such as epidermal growth factor (EGF) and fibronectin. These domains appear to be involved in specific interactions between the proteases and their substrates, cofactors, and/or inhibitors.
Structures of Coagulation Factors
Factors II, VII, IX, and X are homologous to each other at their N-terminal ends. After removal of the signal peptide, a carboxylase residing in the endoplasmic reticulum or Golgi binds to the propeptide region of each of these proteins and converts ~10-12 glutamate (Glu) residues to -carboxyglutamate (Gla) in the adjacent "Gla domain" (Fig. 6). The propeptide is removed from the carboxylated polypeptide prior to secretion. The Gla residues bind calcium ions and are necessary for the activity of these coagulation factors. Synthesis of Gla requires vitamin K. During -carboxylation, vitamin K becomes oxidized and must be reduced subsequently in order for the cycle to continue (Fig. 7). The anticoagulant drug warfarin (Coumadin®) inhibits reduction of vitamin K and thereby prevents synthesis of active factors II, VII, IX, and X.
-Carboxylation of Prothrombin (Factor II)
Role of Vitamin K in Biosynthesis of Factors II, VII, IX, and X
B. Non-enzymatic protein cofactors include factors V and VIII, tissue factor, and high-molecular weight kininogen (HMWK) (Table 3). Factors V and VIII are large plasma proteins that contain repeated sequences homologous to the copper-binding protein ceruloplasmin (A1, A2 & A3 domains in Fig. 8). Thrombin cleaves V and VIII to yield activated factors (Va and VIIIa) that have at least 50 times the coagulant activity of the precursor forms (Fig. 8). Va and VIIIa have no known enzymatic activity. Instead, they serve as cofactors that increase the proteolytic efficiency of Xa and IXa, respectively. Factor VIII circulates in plasma bound to von Willebrand factor; thus, a patient with von Willebrand factor deficiency will generally have a concomitant decrease in the plasma concentration of factor VIII.
Tissue factor is an integral membrane protein that is expressed on the surface of "activated" monocytes, endothelial cells exposed to various cytokines (e.g., tumor necrosis factor), and other cells. It is not found in plasma. Tissue factor greatly increases the proteolytic efficiency of VIIa.
Non-enzymatic Protein Cofactors
Plasma (bound to vWF)
Activation of Factor VIII by Thrombin
C. Prothrombin activation. Factor Xa converts prothrombin (factor II) to thrombin (factor IIa) by cleaving two peptide bonds in the zymogen (indicated by arrows in Fig. 5). Activation of prothrombin by Xa is accelerated by Va, platelets (or phospholipids), and calcium ions (Table 4). The complete system activates prothrombin at a rate about 300,000 times greater than that of Xa and calcium alone. Phospholipids and Va can accelerate prothrombin activation independently (cf. lines 1, 2, and 3 in Table 4), but they act synergistically in the complete system (i.e., 50 x 350 19,000). Rapid activation occurs only when prothrombin and Xa both contain Gla residues and, therefore, have the ability to bind calcium (cf. lines 4 and 5 in Table 4). Binding of calcium alters the conformation the Gla domains of these factors, enabling them to interact with a membrane surface provided by phospholipids in vitro or platelets in vivo (note that platelets are more efficient in this regard). Interactions among the components of the "prothrombinase" complex are shown schematically in Fig. 9. Aggregated platelets are thought to provide the surface upon which prothrombin activation occurs at a site of hemostasis. Activation of factor X by IXa and its cofactor VIIIa appears to occur by a mechanism similar to that of prothrombin activation and may also be accelerated by platelets in vivo.
Acceleration of Prothrombin Activation by Factor Va and Platelets
Relative Rate of
Prothrombin, Xa, Ca++
Prothrombin, Xa, PL, Ca++
Prothrombin, Xa, Va, Ca++
Prothrombin*, Xa*, Va, PL, Ca++
Prothrombin, Xa, Va, PL, Ca++
Prothrombin, Xa, Va, platelets, Ca++
PL = phospholipids
Prothrombin* and Xa* are Gla-less forms of prothrombin and Xa, synthesized in the absence of vitamin K.
Prothrombin Activation Complex
D. Fibrinogen consists of three pairs of polypeptide chains covalently linked by disulfide bonds (Fig. 10) and has a molecular weight of approximately 330,000. Thrombin converts fibrinogen to fibrin monomers by cleaving fibrinopeptides A (16 amino acid residues) and B (14 amino acid residues) from the N-terminal ends of the A and B chains, respectively. Removal of the fibrinopeptides allows the fibrin monomers to form a gel consisting of long polymers as shown in Fig. 11. Formation of the fibrin gel constitutes the end point of the aPTT and PT tests. At this stage, the fibrin monomers are bound to each other non-covalently.
Structure of Fibrinogen
E. Factor XIII. Covalent cross-linking of fibrin polymers by activated factor XIII (XIIIa) is required for adequate clot strength and normal wound healing in vivo. The zymogen form of factor XIII is converted to an active enzyme (XIIIa) by thrombin (Fig. 12). XIIIa catalyzes a transglutamination reaction that initially cross-links the C-terminal ends of the chains on adjacent fibrin monomers. Several other sites become cross-linked more slowly and give the clot additional strength. Because cross-linking of fibrin occurs after formation of a visible clot in vitro, the aPTT and PT tests do not depend on the activity of factor XIII. However, plasma factor XIII can be assayed by determining the solubility of a clot in the presence of urea, which cannot dissolve cross-linked fibrin.
Transglutaminase Activity of Factor XIIIa
F. Amplification and localization of coagulation reactions. The catalytic nature of coagulation reactions allows tremendous amplification of the initial stimulus. Each VIIa/tissue factor complex can produce many Xa molecules, which subsequently produce an even greater amount of thrombin. Amplification also results from positive feedback reactions (see Fig. 13). These include activation of V, VIII, and possibly XI by thrombin, as well as activation of VII by Xa. Assembly of activation complexes on cell membranes normally serves to localize coagulation to sites of vessel injury. Membrane-associated reactions include
(a) activation of X and IX by the VIIa/tissue factor complex on the surface of smooth muscle cells or other cells located beneath the vascular endothelium,
(b) activation of X by the IXa/VIIIa complex on the surface of platelets that have become activated at the site of injury, and
(c) activation of prothrombin by the Xa/Va complex on the surface of activated platelets.
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The Clotting Pathway
Author: Kanchan Ganda, M.D.
Important key words or phrases.
Important concepts or main ideas.
1. The Clotting Pathway
1.1. Elements of Hemostasis:
Platelets (normal: 150- 400,000/μl)
Von Willebrand’s Factor (VWF)
1° (Primary) Hemostasis is the first line of defense forming the primary clot. Platelets, Von Willebrand’s Factor (VWF) and blood vessel response, contributes to 1° hemostatis. Primary hemostasis is reinforced by the formation of a Fibrin clot via clotting factor interractions.
2° (Secondary) Hemostasis is the second line of defense.
1.2. Clotting Cascade: Mechanism of Clot Formation
Time required for the patient to stop bleeding after an injury.
BT (bleeding time) measures the functioning capacity of the platelets.
Normal bleeding time range is: 2-10 minutes.
A normal BT indicates absence of Von Willebrand’s disease and normal platelet function.
It tells us nothing specific about the actual platelet number.
1.5. PT/INR and PTT/APTT
These blood tests check clotting factor function.
Partial thromboplastin time or activated partial thromboplastin time (PTT/APTT) measures the activity of the intrinsic and common pathways.
The normal range of PTT is 25–38 seconds.
This test checks factors XII to I (fibrin). Prothrombin time/ International Normalized Ratio, (PT/INR) measures the extrinsic and the common path-ways.
The normal range of PT is 10–12 seconds. PT/INR check factors VII to I (fibrin).
The PT test has been replaced by the more sophisticated test, the INR.
The normal INR is 0.9-1.2.
The INR was developed to standardize the different preparations of thromboplastin, universally.
Normal INR: 0.9 - 1.2
*International Sensitivity Index of Thromboplastin
INR: 2.0 - 3.0 : It is the Theraputic Range maintained with warfarin sodium
INR under 2.0 is associated with minimal bleeding
INR of 3 - 4.5: Is associated with excessive bleeding
INR is checked every 4-6 weeks
Note: All of the clotting factors except VIII are produced in the liver.
Factor VIII is manufactured in the endothelial cells of the blood vessels.
We see a prolonged PT/INR and PTT/APTT in patients with cirrhosis of the liver (liver failure) when the clotting factor pool has dropped below 50% of normal.
Factors II, VII, IX, and X are vitamin K dependent clotting factors.
Without adequate vitamin K, we see prolonged PT/INR and PTT/APTT values.
Vitamin K is a fat soluble vitamin and needs the presence of bile to be absorbed from the GI tract.
Low bile production, inadequate vitamin K intake, and chronic small intestinal disease can all result in chronic vitamin K deficiency and prolonged PT/INR and PTT/APTT values.
Factor VII has the shortest half-life compared to the other factors.
1.6. Blood thinners
Blood thinners frequently used in patients with an increased tendency toward intravascular clotting or thrombus formation, e.g., deep vein thrombosis, stroke, atrial fibrillation, etc.
In life threatening situations, Heparin sodium is administered intravenously (IV). It is quick acting, has a half-life of one hour, and the drug is completely eliminated in six hours. It is the drug of choice during an emergency (heart attack (MI), CVA (stroke)). Heparin sodium is given IV q6h (every six hours) until the patient is stable. The Warfarin is started next, by mouth. Heparin sodium affects the intrinsic pathway, thus prolonging the PTT/APTT. We check the PTT/APTT to determine how the patient is doing on Heparin sodium. We administer heparin and maintain PTT at 1.5-2.0 times the normal PTT/APTT values ( thus increased to 38-76 seconds). This range of prolongation is referred to as the therapeutic index for Heparin sodium. A PTT/APTT above 76 seconds will result in excessive bleeding.
Warfarin is a vitamin K antagonist and is administered orally. It blocks the function of the vitamin K dependent clotting factors (II, VII, IX, X). Thus large doses of Warfarin affects both the intrinsic and the extrinsic pathways. It takes 9-16 hours for Warfarin effects to show up in the blood after oral intake (PO) and 36 to 48 hours for complete effectiveness, i.e for therapeutic index levels for the drug to occur. Factor VII has the shortest half-life of all factors.
Only PT/INR is measured to establish therapeutic levels of Warfarin activity, instead of both PT/INR and PTT. Excess intake of Warfarin (beyond therapeutic levels) will result in low levels of factors II, IX and X. The patient will then have a prolonged PTT in addition to a prolonged PT/INR. Too much Warfarin will cause spontaneous bleeding in the patient. So at therapeutic levels, only PT/INR is affected. We administer and maintain Warfarin at 1.3- 1.5 times the normal PT values of 10-12 seconds (13-18 seconds). This is the therapeutic index. Warfarin is given only after an emergency situation has been stabilized by Heparin sodium. Once Warfarin is established, the patient then maintains blood thinning by daily oral intake of Warfarin.
All patients on Warfarin get their PT/INR levels done every 6 weeks to confirm that they are in the "therapeutic range." Most patients are maintained at an INR of 2.0-3.0.
Patients with a higher INR of 3.5-4.5 have a greater tendency towards bleeding. (The higher INR is maintained to prevent thrombosis.)