What stops a bleeding cut?
A small cut or wound will stop bleeding due to a process called
coagulation, or thrombogenesis. Coagulation is only possible due to
the presence of certain enzymes in the blood. It is a complicated
process, but this answer will try to make it seem simple.
Blood contains an enzyme called thrombin, which is produced in
the liver. When a cut occurs, the brain sends a signal to the liver
to activate the thrombin, which only becomes active when there is
an open wound. Thrombin joins with a specific protein in blood
plasma called fibrinogen to form incredibly tiny needlelike
crystals called fibrin. The joining of these two substances creates
a biochemical alliance that catches blood cells called corpuscles
as they try to exit the body through the wound. This, in turn,
causes the formation of a plug, or blood clot, which stops the
bleeding.