Thrombi that occur in different parts of the circulation have different causative factors and different macroscopic appearances. Those former in fast-moving blood in arteries and cardiac chambers have a relatively high platelet/fibrin content and very firm and pale,with prominent laminations. Thrombi that form in slow-moving blood,such as that in veins,have a high proportion of trapped red cells relative to fibrin/platelets, and are typically red, soft and gelatinous in texture, with poor laminations.
As most thrombi that form in small or medium-sized vessels occlude the lumen and prevent blood flow, they are termed occlusive. However, thrombi that form in the heart or aorta generally do not cause complete occlusion, appearing as raised plaques applied to the wall (mural thrombi).