If your skin is cut in some way your bleed. This is good to some extent because the bleeding washes away microbes that may have got through your skin. The bleeding cannot be allowed to go on for too long though, because as you bleed you are losing valuable materials that your body has worked to get for you.
Clotting
Very quickly after you are cut a blood clot starts to form. This provides a temporary covering over the wound to allow new cells to seal the cut underneath.
Stages of blood clotting
The clotting process is a complex one, but simplified it involves an enzyme thrombin produced by blood platelets at a damage site changing a soluble protein fibrinogen in the blood into an insoluble protein called fibrin. This fibrin is what forms the mesh in the picture. Red cells then get stuck in this mesh and the whole thing dries pulling the wound together and sealing it.
Haemophilia - some people (usually males) lack one of the blood clotting factors, as a result of a genetic disease. As a result their blood doesn't clot and they are at risk of bleeding to death if injured. Hospitals provide them with the missing factor so they can inject this if it is required.
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