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Introduction to histopathology
Introduction to histopathology

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3.2 Thrombosis and embolism

Blood clots may form in vessels for a variety of reasons. A blood clot is called a thrombus, and the process by which it forms is thrombosis. Embolism occurs when something is carried through the circulation from one site to another. When a thrombus breaks away and is carried through the circulation, it is referred to as a thromboembolism. Other examples of emboli are tumour cells or air-embolism, where air is accidentally introduced into the circulation by a physician. Thromboembolism can block the downstream blood vessels; emboli formed in veins pass through the heart to block arteries in the other side of the circulation, while thrombi formed in arteries can block vessels in the organ where they form. Exactly which vessels may become blocked also depends on the size of the embolism, and the site determines what damage may follow.


(a) If a thrombus is formed in the veins of the leg, where is it likely to end up? (b) If thrombi form on the tricuspid valves of the heart (leading to the aorta), where might the emboli end up?


(a) Thromboemboli formed in leg veins will usually pass through the heart to end up in the pulmonary arterial circulation, causing damage to the lung. (b) Thrombi formed on the tricuspid valves will pass into the systemic circulation and are particularly damaging if they enter the cerebral or carotid arteries, as they can then damage the brain (stroke).