Answer:
If the wall of a blood vessel wall is breached it can be ctritical to repair it before to much blood is lost from the system and all the cells around start to die off. Freely dissolved within the blood plasma is the protein Fibrinogen. This is a structure called a Zymogen (innactive precurser proteins that are cleaved to form activie proteins of enzymes). Fibrinogen is cleaved to form the clotting protein Firbrin. Fibrin is very good at making atractions with itself to form a Mech that covering the breach. This is enough to keep the blood cell in the vessel but the plasma can still easily be lost through the gaps in the mesh. This is where the "cells" come in. These "cells" are in fact not truly cells at all but cell fragments called Platelets. They are small enough to get part way through the mesh but then befome trapped blocking it up. The mesh continues to form more leayer and traps more and more platelets. This collective barrier that is formed is what blocks the blood from escaping the vessel.
It is the red blood cells that block an leaking vessel. For example when you scrap your knee, it bleeds aye? The things called Fibrin, kinda like sticky stuff, snatch moving blood cells and clog the leak so the bleeding stops. The more you exercise the more quickly it can block the leak!
platelets