Wilma Cooper, age 11, of Booneville, Ky., for her question:

HOW DOES AIR FORM A SCAB OVER A WOUND?

Childhood just wouldn't be childhood without a few little scrapes and bruises. Active and curious youngsters usually have a few scars here and there to remind them of the big old tree in the back yard or racing down the hill on skates. Before the scar, of course, came the scab  our body's ingenious way of healing an open, sometimes painful wound.

Any time the skin is broken, the resulting injury is called an open wound. Abrasions are surface wounds where the skin is scraped off. If you cut your finger with a sharp knife, you suffer an incision. A laceration is the tearing of the tissue, and puncture wounds are caused by objects which puncture the skin and underlying tissue. Usually these open wounds bleed for a bit before the blood clots and a scab begins to form.

Clotting at the surface of an open wound starts as platelets  particles in the blood produced in the bone marrow ¬pile up in damaged blood vessels. The platelets and the injured tissue of the vessels give off certain chemicals which react with clotting factors in the plasma (the liquid part of blood) to form a substance called thromboplastin. At this point certain inactive blood factors undergo changes to form long, sticky threads of fibrin. This clogs and thickens the blood and slows down the flow from the severed vessels. As the spongy wad of fibrin thickens, it forms a clot which plugs up the open wound and stops the bleeding. The dryness of the air helps the clot to harden to a scab. This protects the wound during the patient repair work that rebuilds the damaged tissues below the surface.

An open wound should be carefully cleaned and covered for a day or so to prevent germ invasion. Strong antiseptics should be avoided as they may damage the tissues. After a scab has formed  it should be allowed to fall off naturally  not be pried off with impatient fingers.

Some people have a shortage of one of the blood¬ clotting factors. This prevents their blood from clotting properly and is a very serious condition where even the smallest wound can be extremely dangerous. Fortunately, most people's blood clots normally, a scab forms and the miraculous process of healing occurs.