Clot

A clot, in medical terms, is a semi-solid mass formed in blood to stop bleeding or to occlude a vessel. It mainly comprises fibrin strands, platelets, and trapped red blood cells. Clot formation (hemostasis) begins with vascular constriction, followed by platelet plug formation and a coagulation cascade that converts fibrinogen to fibrin, stabilizing the plug and supporting tissue repair.

The coagulation cascade has intrinsic and extrinsic pathways that converge on a common pathway generating thrombin, which converts fibrinogen into fibrin and activates more platelets. Natural anticoagulants regulate thrombin and limit clot growth to prevent excessive thrombosis.

Dissolution occurs through fibrinolysis, in which plasmin breaks down fibrin. This process is triggered after vessel repair and is aided by tissue plasminogen activator. A balance between clot formation and dissolution is essential; disruption can cause bleeding or thrombosis.

Pathological clots within vessels can cause venous thromboembolism, pulmonary embolism, stroke, or heart attack. Risk factors include prolonged immobility, surgery, cancer, obesity, smoking, and inherited clotting disorders. Treatments include anticoagulants, antiplatelet drugs, thrombolytics, and sometimes mechanical clot removal or compression therapy.

In everyday language, a clot may also refer to a viscous mass that forms in liquids, such as clay or wax. In medicine, the term most often denotes a blood clot.