Blood clot. Colored enhanced scanning electron micrograph (SEM) of a blood clot in a human artery. The clot consists of threads of fibrin (yellow), an insoluble protein. These form a mesh, which traps blood cells (red) and seals the damaged blood vessel to prevent bleeding. A column of stacked single red blood cells, known as a "rouleaux formation", is shown at the top center. Blood clots are triggered by chemicals released by platelet cells in the blood or by tissue cells at the injury site. These chemicals begin a complex series of chemical reactions in the blood, which result in the formation of fibrin. An abnormal clot (thrombus) can also occur in intact blood vessels, and is the principal cause of heart attack and stroke. Magnification: x1,900 at 5x7cm size. X950 at 35mm size.