by PhilipJ on 2 November 2006
When you cut yourself, you bleed, but the bleeding rapidly stops. Blood has a built-in emergency repair system that quickly blocks any damage to the circulatory system, creating a temporary patch that allows time for more permanent repairs. Three basic mechanisms are at work. First, platelets (small fragments of blood cells that circulate in the blood) clump at the site of the wound, forming a weak plug. Second, neighboring blood vessels constrict, reducing the amount of blood flowing into the area. Finally, the protein fibrin assembles into a tough network that clots the blood and forms an insoluble blockage. Together, these methods stop the loss of blood and create a sturdy scab to protect the area as you heal.
Fibrin is normally present in an inactive form known as fibrinogen. Fibrinogen is soluble in water and is found in high concentrations in the blood, where it waits until needed to form a clot. When given the signal, fibrinogen in converted to fibrin, which then assembles into an extended network of fibers. This changes the normally-fluid blood into a jelly-like solid, which then dries to form a scab. Of course, it is very important to assemble fibrin networks only in the local area of the cut, and nowhere else, since the blood must continue flowing to other parts of the body.