Coagulation

When the body has to stop blood from getting out, it sets hemostasis in motion, the series of steps that close off the injury. Platelets arrive and pile up until they build a plug, but a heap of stacked platelets does not hold together on its own.

That is where clotting comes in, producing a substance called fibrin. Fibrin does not stay loose: it settles into a mesh, crisscrossed threads like the net thrown over a load so it will not shift on the road. That net is what keeps the platelet plug right there, at the site of the injury.