A thrombus, colloquially called a grume, is that the final product of the blood clotting step in hemostasis. There are two components to a thrombus: aggregated platelets and red blood cells that form a plug, and a mesh of cross-linked fibrin protein. The substance making up a thrombus is sometimes called a cruor. A thrombus may be a healthy response to injury intended to stop bleeding but are often harmful in thrombosis when clots obstruct blood flow through healthy blood vessels.
Mural thrombi are thrombi that adhere to the wall of a vessel. They occur in large vessels like the guts and aorta and may restrict blood flow but usually don't block it entirely. They appear grey-red with alternating light and dark lines (known as lines of Zahn) which represent bands of entrapped white blood cells and red blood cells (darker).