We call it this because it goes on to activate prothrombin into its active form known as Thrombin. And once again, it's important to keep in mind that all these enzymes are circulating in close proximity, they're found floating around inside our blood. Now, as soon as we form Thrombin, thrombin goes three different ways. It does three important things. It basically calls upon platelets. Now, platelets are these pieces of megacaryocytes that are also used to form that meshlike network of blood clots that creates that seal and prevents that movement of liquid of blood out of that blood vessel.