Blood clotting factors

Overview of blood clotting factors and their role in hemostasis

Blood clotting factors are a group of proteins essential for the coagulation of blood, a process that prevents excessive bleeding when the vascular system is injured. These factors work in a complex cascade to form a stable blood clot. The coagulation cascade is traditionally divided into the intrinsic pathway, extrinsic pathway, and common pathway.

Overview

Blood clotting factors are primarily produced by the liver and circulate in the bloodstream in an inactive form. Upon vascular injury, these factors are activated in a specific sequence, leading to the formation of a fibrin clot. The main clotting factors are numbered I through XIII, and each plays a unique role in the coagulation process.

Key Clotting Factors

• Factor I (Fibrinogen): A soluble plasma glycoprotein that is converted by thrombin into fibrin during clot formation.
• Factor II (Prothrombin): A vitamin K-dependent protein that is converted into thrombin, which then converts fibrinogen to fibrin.
• Factor III (Tissue Factor): A protein that initiates the extrinsic pathway by forming a complex with Factor VIIa.
• Factor IV (Calcium ions): Essential for various steps in the coagulation cascade.
• Factor V (Proaccelerin): Acts as a cofactor in the conversion of prothrombin to thrombin.
• Factor VII (Proconvertin): Initiates the extrinsic pathway by activating Factor X.
• Factor VIII (Antihemophilic Factor): A cofactor for Factor IX, essential for the intrinsic pathway.
• Factor IX (Christmas Factor): A vitamin K-dependent protein that activates Factor X.
• Factor X (Stuart-Prower Factor): Converts prothrombin to thrombin in the common pathway.
• Factor XI (Plasma Thromboplastin Antecedent): Activates Factor IX in the intrinsic pathway.
• Factor XII (Hageman Factor): Initiates the intrinsic pathway.
• Factor XIII (Fibrin Stabilizing Factor): Cross-links fibrin, stabilizing the clot.

Coagulation Pathways

The coagulation cascade is divided into three pathways:

Intrinsic Pathway

The intrinsic pathway is activated by trauma inside the vascular system and involves factors XII, XI, IX, and VIII. It is measured by the activated partial thromboplastin time (aPTT) test.

Extrinsic Pathway

The extrinsic pathway is activated by external trauma that causes blood to escape from the vascular system. It involves Factor VII and is measured by the prothrombin time (PT) test.

Common Pathway

The common pathway begins with the activation of Factor X and leads to the conversion of prothrombin to thrombin and fibrinogen to fibrin, forming a stable clot.

Clinical Significance

Deficiencies or dysfunctions in clotting factors can lead to bleeding disorders such as hemophilia and von Willebrand disease. Conversely, excessive clotting can result in conditions like deep vein thrombosis and pulmonary embolism.

Also see

• Hemostasis
• Platelet
• Thrombosis
• Anticoagulant
• Vitamin K