What makes blood clot?

If our blood failed to clot after an injury we would die of excessive bleeding. Clotting is a natural mechanism to protect us from this danger. Clotting is made possible by a substance called fibrin. Threads of fibrin hold the blood cells together in a clot. Blood clots at different rates in different people. The clotting process will begin only in the presence of a rough surface or if there is an injury to blood vessels.