Randy Arpia, age 12, of San Diego, California, for his question:

What makes a cut stop bleeding?

    This remarkable happening is one of the body's many talents for healing itself. The healing of a cut often seems like a long slow process, but the cleverest medical scientist cannot explain the miraculous stages in detail. The first step, naturally, is to stop the bleeding. The circulating bloodstream carries special chemicals in readiness to meet this emergency. When the flesh is wounded by a cut, these factors spring into action.        

    Some of them are carried in the liquid plasma, others in floating blood cells called platelets. When exposed to the air, special chemicals in the plasma and the platelets react together to form fibrous material called fibrin. This clogs and thickens the blood and slows down the flow from the severed vessels. As the spongy wad thickens it forms a clot that plugs up the open cut and stops the bleeding. The clot becomes a scab to protect the wound during the patient repair work that rebuilds the damaged tissues below the surface.