Components of Blood Coagulation Reactions

Factor Number or Name	Synonym	Purpose
Plasma factors
I	Fibrinogen	A precursor of fibrin monomers (and polymers)
II	Prothrombin	The precursor of thrombin Thrombin converts fibrinogen to fibrin; activates soluble factors V, VIII, XI, and XIII; and binds to thrombomodulin to activate protein C Is vitamin K–dependent
V	Proaccelerin	Is activated to factor Va, a cofactor for the enzyme factor Xa in the factor Xa/factor Va/phospholipid complex, which cleaves prothrombin to thrombin Is present in alpha granules in platelets Factor Va inactivated by activated protein C in complex with free protein S
VII	Proconvertin	Binds to tissue factor and is then activated to form the enzymatic component of the factor VIIa/tissue factor complex, which activates factor X and, possibly, factor IX Is vitamin K–dependent
VIII	Antihemophilic globulin	Is activated to factor VIIIa, a cofactor for the enzyme factor IXa in the factor IXa/factor VIIIa/phospholipid complex, which activates factor X Is a large cofactor protein (as is factor V) Is secreted bound to von Willebrand factor multimers from the Weibel-Palade bodies of endothelial cells (factor VIII also circulates with von Willebrand factor) As factor VIIIa, is inactivated by activated protein C in complex with free protein S (as is factor Va)
IX	Christmas factor	Is activated to factor IXa, the enzyme of the factor IXa/factor VIIIa/phospholipid complex, which activates factor X Is vitamin K–dependent
X	Stuart-Prower factor	Is activated to factor Xa, the enzyme of the factor Xa/factor Va/phospholipid complex, which cleaves prothrombin to thrombin Is vitamin K–dependent
XI	Plasma thromboplastin antecedent	Is activated to factor XIa, which may activate factor IX
Prekallikrein	Fletcher factor	Participates in a reciprocal reaction in which it is activated to kallikrein by factor XIIa As kallikrein, catalyzes further activation of factor XII to factor XIIa Circulates as a biomolecular complex with high molecular weight kininogen
High molecular weight kininogen	Fitzgerald factor	Circulates as a bimolecular complex with prekallikrein
XII	Hageman factor	When activated to factor XIIa by surface contact, kallikrein, or other factors, it activates prekallikrein and factor XI, triggering the intrinsic coagulation pathway in vitro
XIII	Fibrin stabilizing factor	When activated by thrombin, catalyzes formation of peptide bonds between adjacent fibrin monomers to strengthen and stabilize fibrin clots
Protein C	—	Is activated by thrombin bound to surface membrane-bound thrombomodulin (CD141); then proteolyzes and inhibits (in the presence of free protein S and phospholipid) the cofactor activity of factor VIIIa and factor Va Is vitamin K–dependent
Protein S	—	Circulates in plasma as free protein S and as protein S bound to C4b-binding protein of the complement system Functions in its free form as a cofactor for activated protein C Is vitamin K–dependent
Cell surface factors
Tissue factor	Tissue thromboplastin	Is a protein that is present in the membranes of certain tissue cells, including perivascular fibroblasts, endothelial cells, boundary epithelial cells (eg, epithelial cells of the skin, amnion, and gastrointestinal and genitourinary tracts), glial cells of the nervous system, monocytes, macrophages, and some tumor cells It is exposed to flowing blood during injury or inflammation, binds factor VIIa, and initiates the extrinsic coagulation pathway
Procoagulant phospholipid	—	Acidic phospholipid (primarily phosphatidyl serine) present on the surface of activated platelets, endothelial cells, and other tissue cells Is a component of the factor IXa/VIIIa/phospholipid complex which activates factor X and of the factor Xa/factor Va/phospholipid complex which activates prothrombin
Thrombomodulin	CD141	Is an endothelial cell surface binding site for thrombin which, when bound to thrombomodulin, activates protein C

Factor Number or Name

Synonym

Purpose

Plasma factors

Fibrinogen

A precursor of fibrin monomers (and polymers)

Prothrombin

The precursor of thrombin

Thrombin converts fibrinogen to fibrin; activates soluble factors V, VIII, XI, and XIII; and binds to thrombomodulin to activate protein C

Is vitamin K–dependent

Proaccelerin

Is activated to factor Va, a cofactor for the enzyme factor Xa in the factor Xa/factor Va/phospholipid complex, which cleaves prothrombin to thrombin

Is present in alpha granules in platelets

Factor Va inactivated by activated protein C in complex with free protein S

VII

Proconvertin

Binds to tissue factor and is then activated to form the enzymatic component of the factor VIIa/tissue factor complex, which activates factor X and, possibly, factor IX

Is vitamin K–dependent

VIII

Antihemophilic globulin

Is activated to factor VIIIa, a cofactor for the enzyme factor IXa in the factor IXa/factor VIIIa/phospholipid complex, which activates factor X

Is a large cofactor protein (as is factor V)

Is secreted bound to von Willebrand factor multimers from the Weibel-Palade bodies of endothelial cells (factor VIII also circulates with von Willebrand factor)

As factor VIIIa, is inactivated by activated protein C in complex with free protein S (as is factor Va)

Christmas factor

Is activated to factor IXa, the enzyme of the factor IXa/factor VIIIa/phospholipid complex, which activates factor X

Is vitamin K–dependent

Stuart-Prower factor

Is activated to factor Xa, the enzyme of the factor Xa/factor Va/phospholipid complex, which cleaves prothrombin to thrombin

Is vitamin K–dependent

Plasma thromboplastin antecedent

Is activated to factor XIa, which may activate factor IX

Prekallikrein

Fletcher factor

Participates in a reciprocal reaction in which it is activated to kallikrein by factor XIIa

As kallikrein, catalyzes further activation of factor XII to factor XIIa

Circulates as a biomolecular complex with high molecular weight kininogen

High molecular weight kininogen

Fitzgerald factor

Circulates as a bimolecular complex with prekallikrein

XII

Hageman factor

When activated to factor XIIa by surface contact, kallikrein, or other factors, it activates prekallikrein and factor XI, triggering the intrinsic coagulation pathway in vitro

XIII

Fibrin stabilizing factor

When activated by thrombin, catalyzes formation of peptide bonds between adjacent fibrin monomers to strengthen and stabilize fibrin clots

Protein C

—

Is activated by thrombin bound to surface membrane-bound thrombomodulin (CD141); then proteolyzes and inhibits (in the presence of free protein S and phospholipid) the cofactor activity of factor VIIIa and factor Va

Is vitamin K–dependent

Protein S

—

Circulates in plasma as free protein S and as protein S bound to C4b-binding protein of the complement system

Functions in its free form as a cofactor for activated protein C

Is vitamin K–dependent

Cell surface factors

Tissue factor

Tissue thromboplastin

Is a protein that is present in the membranes of certain tissue cells, including perivascular fibroblasts, endothelial cells, boundary epithelial cells (eg, epithelial cells of the skin, amnion, and gastrointestinal and genitourinary tracts), glial cells of the nervous system, monocytes, macrophages, and some tumor cells

It is exposed to flowing blood during injury or inflammation, binds factor VIIa, and initiates the extrinsic coagulation pathway

Procoagulant phospholipid

—

Acidic phospholipid (primarily phosphatidyl serine) present on the surface of activated platelets, endothelial cells, and other tissue cells

Is a component of the factor IXa/VIIIa/phospholipid complex which activates factor X and of the factor Xa/factor Va/phospholipid complex which activates prothrombin

Thrombomodulin

CD141

Is an endothelial cell surface binding site for thrombin which, when bound to thrombomodulin, activates protein C

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Overview of Hemostasis