Hyytymisreitistö

Hyytymisreitistö, often referred to as the coagulation cascade in English, is a complex series of biochemical reactions that leads to the formation of a blood clot. This process is crucial for hemostasis, the prevention of blood loss from a damaged blood vessel. The cascade involves a series of inactive enzyme precursors called zymogens, which are sequentially activated by proteolysis, ultimately leading to the conversion of fibrinogen into fibrin.

There are two main pathways within the coagulation cascade: the intrinsic pathway and the extrinsic pathway. Both pathways converge on a common pathway. The intrinsic pathway is initiated by contact activation within the blood vessel, involving factors like Factor XII. The extrinsic pathway is triggered by tissue damage, releasing tissue factor (Factor III), which then initiates a different set of reactions.

Regardless of the initiating pathway, the common pathway involves the activation of Factor X. Activated Factor X, along with Factor V, calcium ions, and phospholipids, forms the prothrombinase complex. This complex catalyzes the conversion of prothrombin (Factor II) into thrombin (Factor IIa). Thrombin is a pivotal enzyme in the cascade; it converts soluble fibrinogen into insoluble fibrin strands, which then polymerize to form a mesh. This fibrin mesh traps blood cells and platelets, creating a stable clot.

The coagulation cascade is tightly regulated to prevent excessive clotting, which could lead to thrombosis. Inhibitory mechanisms, such as antithrombin and protein C, help to control the activity of the coagulation factors. Dysfunctions in the hyytymisreitistö can lead to bleeding disorders or thrombotic diseases.