Clots

Clots are semi-solid masses formed by the coagulation of blood. In the circulatory system, they contribute to hemostasis by sealing vessel injuries and limiting blood loss. A blood clot consists mainly of a fibrin network, platelets, red blood cells, and white blood cells. When a vessel is damaged, vessels constrict and platelets adhere to exposed tissue, forming a plug. Coagulation factors activate thrombin, which converts fibrinogen into fibrin. The fibrin mesh solidifies the plug, trapping cells. After healing, the clot is removed by fibrinolysis, primarily through plasmin.

Pathologic clots can obstruct vessels. A clot that travels is an embolus, which may cause pulmonary embolism, stroke, or heart attack. Thrombosis can occur in arteries or veins; risk factors include immobility, surgery, cancer, pregnancy, genetic clotting disorders, smoking, and certain medications. Virchow’s triad—stasis, hypercoagulability, and endothelial injury—summarizes common contributors to clot formation.

Clinicians diagnose clots using a combination of clinical assessment, imaging studies, and laboratory tests. D-dimer tests can indicate active clot breakdown but are not specific. Treatments include anticoagulants to prevent new clots, antiplatelet drugs for arterial clots, thrombolytics in certain cases, and mechanical clot removal if necessary. Prevention emphasizes mobility, hydration, compression devices, and, when indicated, prophylactic anticoagulation in high-risk patients.