This scanning electron micrograph shows the fibrous material that makes a blood clot, magnified 4,000 times. It has been coloured to show the disc-shaped red blood cells and the meshwork of the protein fibrin more clearly. When tissue is damaged, a reaction in the blood converts a soluble protein called fibrinogen into insoluble fibrin. A fibrin mesh grows and traps blood cells, forming a blood clot that seals off broken blood vessels and stops further blood loss.