Thrombosis results from disruption of Virchow's triad: blood stasis, vessel wall injury, or hypercoagulability. Pathologic clots obstruct blood flow and cause ischemia, inflammation, and tissue necrosis.
Examine how each component of the triad contributes to clot formation: cardiac arrhythmias cause stasis; atherosclerotic plaques expose collagen; malignancy elevates tissue factor. Study the distinction between white clots (arterial, platelet-rich) and red clots (venous, fibrin-rich).
Not all thrombi are occlusive—some are mural and do not obstruct flow initially. The hypercoagulable state is not inherent to platelets; it involves altered coagulation cascade balance.
Virchow's triad is one of medicine's most enduring frameworks because it elegantly maps three completely different pathological processes onto a single outcome: clot formation where it should not occur. From your hemostasis prerequisites, you know that coagulation is normally a carefully balanced system — platelets adhere to exposed subendothelial collagen, the coagulation cascade amplifies and stabilizes the clot with fibrin, and natural anticoagulants (antithrombin, protein C/S, TFPI) limit the response to the injury site. Thrombosis is what happens when one or more components of this balance tip in favor of clot formation inappropriately.
The first arm of the triad, vessel wall injury, directly exposes the collagen and tissue factor that would normally be hidden beneath intact endothelium. In the arterial circulation, the dominant culprit is atherosclerotic plaque rupture: a vulnerable plaque's fibrous cap tears, exposing its lipid-rich core — which is extraordinarily thrombogenic because it contains oxidized lipids and tissue factor from foam cell macrophages. The resulting platelet-rich white thrombus can occlude a coronary artery within minutes, causing myocardial infarction. This is the mechanism behind most acute MI events, even in patients whose arteries were not critically narrowed before the rupture.
Stasis operates through a subtler mechanism. Blood flow through healthy vessels creates laminar shear forces that sweep activated clotting factors away from the vessel wall and keep platelets suspended in the center of the stream. When flow slows — from atrial fibrillation, venous valve incompetence, prolonged immobility, or obstruction — this sweeping action fails. Activated thrombin, factor Xa, and tissue factor accumulate locally. The coagulation cascade runs to completion without natural anticoagulants keeping pace, producing fibrin-rich red thrombi — so named because red blood cells become entrapped in the fibrin mesh. Venous thromboembolism (deep vein thrombosis, pulmonary embolism) is predominantly a stasis-driven, fibrin-rich clot. This distinction matters clinically: anticoagulants are highly effective against red clots (they block the cascade), while antiplatelets are more effective against white clots (they interrupt platelet aggregation at the plaque rupture site).
Hypercoagulability encompasses inherited and acquired states where the natural anticoagulant balance is disrupted. Factor V Leiden — a mutation that makes factor Va resistant to inactivation by protein C — is the most common inherited thrombophilia, present in ~5% of European populations. Malignancy-associated hypercoagulability (Trousseau's syndrome) occurs because many tumors constitutively express tissue factor, activating the extrinsic pathway systemically. Antiphospholipid syndrome involves antibodies that paradoxically activate coagulation proteins despite "anti"-phospholipid name. The clinical implication is that thrombotic risk is often multiplicative: a woman with Factor V Leiden mutation who takes oral contraceptives (which decrease protein S and increase fibrinogen) has a risk 30–50 times that of the baseline population — far greater than either factor alone. Identifying which arm of Virchow's triad is operative guides both risk stratification and the choice of preventive or therapeutic intervention.