Blood clotting
Introduction
Blood clotting, also known as coagulation, is a complex process that prevents excessive bleeding when a blood vessel is injured. This process involves a cascade of chemical reactions that result in the formation of a clot, a gel-like mass of blood cells and fibrin strands that seal the injured vessel.
Physiology of Blood Clotting
Blood clotting is a multi-step process involving more than 20 different proteins, known as clotting factors. These factors are normally inactive in the bloodstream, but when a blood vessel is damaged, they are activated in a series of reactions known as the coagulation cascade.
The Coagulation Cascade
The coagulation cascade is divided into two pathways: the intrinsic pathway and the extrinsic pathway. Both pathways lead to the common pathway, which results in the formation of a blood clot.
Intrinsic Pathway
The intrinsic pathway is initiated by damage to the blood vessel itself. This damage exposes collagen, a protein found in the vessel wall, which triggers the activation of Factor XII. This in turn activates Factor XI, then Factor IX, and finally Factor VIII, each step catalyzed by a specific enzyme.
Extrinsic Pathway
The extrinsic pathway is initiated by damage to the tissues surrounding the blood vessel. This damage releases a protein called tissue factor, which combines with Factor VII to activate Factor X.
Common Pathway
Both the intrinsic and extrinsic pathways converge at the common pathway, where Factor X activates prothrombin to form thrombin. Thrombin then converts fibrinogen, a soluble protein, into insoluble fibrin strands. These strands form a mesh that traps blood cells, forming a clot.
Regulation of Blood Clotting
The body regulates blood clotting to ensure that clots form only at the site of injury and do not grow too large. This is achieved through several mechanisms, including the action of anticoagulant proteins and the fibrinolytic system.
Anticoagulant Proteins
Anticoagulant proteins, such as antithrombin and protein C, inhibit the action of clotting factors to prevent excessive clotting. Antithrombin binds to thrombin and Factor Xa, inactivating them, while protein C inactivates Factors Va and VIIIa.
Fibrinolytic System
The fibrinolytic system breaks down fibrin strands to dissolve clots once the injury has healed. The main enzyme in this system is plasmin, which is formed from plasminogen by the action of tissue plasminogen activator (tPA).
Disorders of Blood Clotting
Disorders of blood clotting can lead to excessive bleeding or excessive clotting, both of which can be life-threatening. These disorders can be caused by defects in clotting factors or by imbalances in the regulation of clotting.
Hemophilia
Hemophilia is a genetic disorder characterized by a deficiency of Factor VIII (hemophilia A) or Factor IX (hemophilia B). This leads to prolonged bleeding and spontaneous bleeding into joints and muscles.
Thrombophilia
Thrombophilia is a condition characterized by an increased tendency to form clots. This can be caused by genetic mutations that increase the activity of clotting factors or decrease the activity of anticoagulant proteins.