How does blood clot?

Blood travels throughout your body in blood vessels called arteries, veins and capillaries. You can see veins if you look at the back of your hand. They appear bluish in color. Your heart pumps blood through these blood vessels to all the cells of your body. Blood brings food and oxygen to your cells, and removes wastes.

When blood vessels are cut or torn, you bleed. You can bleed externally (outside your skin) or intemally (inside your body). When you break a blood vessel in your skin, a bruise may be created as escaped blood leaks toward the surface of your skin.

If you looked under a microscope, you would see that blood is actually made up of several parts: plasma, a watery, yellowish fluid; red blood cells, the cells that carry food and oxygen to nourish all cells of the body; white blood cells, the cells that attack viruses and germs in the bloodstream; and platelets, round, sticky cells that help in blood clotting. 

When a blood vessel is torn...

	1. The blood vessel lightens...
	2. Sticky platelets plug the hole...
	3. A fibrin net seals the platelet plug in place. The hole is plugged. The bleeding stops.

How does the body know to make a fibrin net? The plasma contains at least 14 substances - proteins - called clotting factors that are needed to clot blood. These proteins are named by roman numerals, for example, factor I and factor II. Each clotting factor has a specific job. The proteins work as a team, in a specific order, one giving directions to the next. It's like a relay race, with one factor handing instructions to another until the last factor, factor VIII, is given the instructions to make the fibrin net.

Externally, the fibrin net holding the platelets in place will appear as a scab. Underneath the scab, new skin starts to grow, to repair the damage. After about a week, the scab will fall off, leaving fresh new skin.

Adapted from Tell Them the Facts! By Laureen A. Kelley, 1995