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[thrombo- + -sis]
The formation or presence of a blood clot within the vascular system. This is a life-saving event when it occurs during hemorrhage. It is a life-threatening event when it occurs at any other time because the clot can occlude a vessel and stop the blood supply to an organ or a part. The thrombus, if detached, can travel through the bloodstream and occlude a vessel distant from the original site, e.g., a clot in the leg may break off and cause a pulmonary embolus.

Trauma (particularly after an operation and parturition), cardiac and vascular disorders, obesity, hereditary thrombophilias (coagulation disorders), age over 65, an excess of erythrocytes and of platelets, an overproduction of fibrinogen, and sepsis are predisposing causes.

Lungs: Obstruction of the smaller vessels in the lungs causes an infarct that may be accompanied by sudden pain in the side of the chest, similar to pleurisy; also present are the spitting of blood, a pleural friction rub, and signs of consolidation. Kidneys: Blood appears in the urine. Skin: Small hemorrhagic spots may appear in the skin. Spleen: Pain is felt in the left upper abdomen. Extremities: If a large artery in one of the extremities, such as the arm, is suddenly obstructed, the part becomes cold, pale, bluish, and the pulse disappears below the obstructed site. Gangrene of the digits or of the whole limb may ensue. The same symptoms may be present with an embolism.

Pathological clots are treated with thrombolytic agents (such as streptokinase), antiplatelet drugs (such as heparins or aspirin), anticoagulants (such as warfarin), or platelet glycoprotein receptor antagonists (such as abciximab). When a thrombus or embolus is large and life threatening, surgical removal may be attempted.

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