What causes blood clots

Blood Clot Formation

Blood clots are masses of coagulated blood that form when platelets and fibrin strands aggregate. While clotting is essential for wound healing and hemostasis, pathological clots can obstruct blood vessels and cause serious complications including pulmonary embolism, stroke, and deep vein thrombosis. Understanding clot formation mechanisms is crucial for identifying risk factors and preventing potentially fatal complications.

Virchow's Triad and Thrombosis

Virchow's triad describes three primary mechanisms promoting thrombosis: blood stasis (slowed flow), vessel wall injury, and hypercoagulability. Most clinical thromboses result from combinations of these factors. Prolonged immobility represents pure stasis, while smoking and atherosclerosis cause endothelial damage. Hypercoagulable disorders, whether inherited or acquired, predispose to clot formation even with minor flow disruption.

Immobility and Venous Stasis

Extended periods without movement dramatically increase deep vein thrombosis risk. During long airplane flights, leg muscle inactivity allows venous blood to pool in calf muscles. Hospital bed rest similarly reduces muscle pump activity necessary for venous return. Immobility during prolonged car drives produces identical mechanisms. Even 4-6 hours of immobility noticeably increases clot risk, with longer periods exponentially elevating risk.

Surgery and Trauma

Surgical procedures trigger clot formation through multiple mechanisms. Direct vessel trauma during surgery activates platelets and the coagulation cascade. Tissue damage releases tissue factor, initiating the extrinsic pathway. Post-operative immobilization, inflammation, and increased blood viscosity from dehydration compound clot risk. Major orthopedic surgeries (hip replacement, knee surgery) show particularly high thromboembolism rates, often reaching 40-60% without prophylaxis.

Cardiovascular Risk Factors

Smoking damages endothelial cells lining blood vessels, exposing underlying collagen that activates platelets. This explains why smokers have doubled stroke and heart attack risk. Atherosclerotic plaques create turbulent flow and vessel wall disruption, promoting clot formation. Atrial fibrillation creates cardiac stasis, allowing clots to form in the left atrial appendage. Cardiac dysfunction and reduced ejection fraction both increase thrombotic risk substantially.

Pregnancy and Hormonal Factors

Pregnancy creates a hypercoagulable state through multiple mechanisms. Increased estrogen stimulates hepatic production of clotting factors VII, VIII, IX, and X. Pregnancy-induced compression of the inferior vena cava by the enlarged uterus increases venous pressure in lower extremities. Tissue factor expression increases, and fibrinolytic activity decreases. These changes persist into the postpartum period, with thrombotic risk remaining elevated for weeks after delivery. Oral contraceptives similarly increase clotting factor levels and elevate thrombotic risk.

Additional Risk Factors

Cancer increases thrombotic risk through tumor-derived procoagulant activity and increased inflammation. Obesity contributes through multiple mechanisms including inflammation, reduced fibrinolysis, and increased clotting factors. Chronic kidney disease impairs fibrinolysis and increases inflammation. Antiphospholipid syndrome, lupus, and other autoimmune conditions create severe hypercoagulability. Advanced age increases clot risk through accumulated endothelial dysfunction and reduced fibrinolytic capacity.