The aggregated platelets swell and disintegrate and are gradually replaced by fibrin.
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6 Leukocytes are attracted by chemotactic factors released from aggregated platelets 2 or proteolytic fragments of plasma proteins and become incorporated into the thrombi. As thrombi age, they undergo progressive structural changes. 4 5 When a platelet-rich arterial thrombus becomes occlusive, stasis occurs, and the thrombus can propagate as a red stasis thrombus. Thrombi that form in regions of slow to moderate flow are composed of a mixture of red cells, platelets, and fibrin and are known as mixed platelet-fibrin thrombi. 3 4 5 In contrast, venous thrombi form in areas of stasis and are predominantly composed of red cells, with a large amount of interspersed fibrin and relatively few platelets. 1 2 Arterial thrombi form under conditions of high flow and are composed mainly of platelet aggregates bound together by thin fibrin strands. Because the relative proportion of cells and fibrin depends on hemodynamic factors, the proportions differ in arterial and venous thrombi. Thrombi are composed of fibrin and blood cells and may form in any part of the cardiovascular system, including veins, arteries, the heart, and the microcirculation.
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