Activated platelets. Coloured scanning electron micrograph (SEM) of activated platelets (yellow) attached to surgical gauze. Platelets are tiny blood cells that help the body form clots to stop bleeding. If a blood vessel gets damaged, it sends out signals that are picked up by platelets. The platelets then rush to the site of damage and form a plug, or clot, to repair the damage. The process of spreading across the surface of a damaged blood vessel to stop bleeding is called adhesion. This is because when platelets get to the site of the injury, they grow sticky tentacles that help them adhere. They also send out chemical signals to attract more platelets to pile onto the clot in a process called aggregation. Magnification: x2000 at 10cm wide.