SEM of a blood clot forming, 2 in a series of 3. A thrombus is the final product of the blood coagulation step in hemostasis. There are two components to a thrombus: aggregated platelets that form a platelet plug, and a mesh of cross-linked fibrin protein. The substance making up a thrombus is sometimes called cruor. A thrombus is a healthy response to injury intended to prevent bleeding, but can be harmful in thrombosis, when clots obstruct blood flow through healthy blood vessels. Magnification unknown.