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How does hemostasis stop bleeding after a blood vessel is injured?
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bleeding and blood clotting
Red blood cells (erythrocytes) trapped in a mesh of fibrin threads. Fibrin, a tough, insoluble protein formed after injury to the blood vessels, is an essential component of blood clots.
Hemostasis is the vital physiological process that arrests bleeding following an injury to a blood vessel. It functions through a highly coordinated sequence of four primary stages:1
- Vasoconstriction: Upon injury, the blood vessel wall contracts, narrowing the vessel to minimize blood loss and reduce the pressure at the site of the breach.213
- Platelet Plug Formation: Platelets adhere to the exposed collagen and to each other at the injury site, forming a temporary mass known as a platelet plug. This process is essential for stopping bleeding from smaller vessels.214
- Coagulation: This secondary phase involves a complex cascade of coagulation factors. Through the extrinsic or intrinsic pathways, the enzyme thrombin is generated, which converts the soluble protein fibrinogen into insoluble fibrin threads. These threads create a mesh that traps blood cells and platelets, forming a resilient blood clot that can withstand the force of blood flow.23
- Fibrinolysis: Once the wound has healed, the clot is eventually dissolved by the enzyme plasmin and replaced by permanent tissue.1