Why does a scab form on a cut?
When the roof leaks, you patch it up to stop the rain from dripping into the house. When the water pipe breaks, you patch it up to stop the water from slopping all over the floor. When you cut yourself, the red red blood oozes out of the veins. The crusty brown scab that forms over the wound is a patch to stop the precious blood from spilling out of your body.
The blood vessels are tubes that branch all through the body. They connect together like a network of highways, big ones and small ones. The blood is always flowing to and from the heart and the heart keeps pumping away to 60ud the blood circulating around and around through the branching tubes. All this network of blood vessels is covered by the skin which cushions them with a soft pad and protects them from harm.
All goes well until the skin is cut or scratched. If the wound is deep enough, it cuts into the tiny blood vessels just under the skin. They break and out oozes the blood. This will not do at all because each tiny little blood vessel is linked to all the other blood vessels. If the little tubes are not patched, all the blood would flow out of the body,
The important patching job is done when a scab forms over the wound. When the cut is new, the blood flows fast and freely. Then the blood becomes thicker and flows more slowly. It gets thicker and darker and a clot forms over the cut. The patching ,fob has started. The thick, clotted blood seals up the broken blood vessels and the bleeding stops.
A cut is likely to become infected, so you apply a germ killer such as iodine to the cut, even if it hurts. Meantime the clotted blood has formed a dark red plug over the wound and the body already has started to repair the damage. Now is the time to stick a band aid over the wound, but this is not because the body has not made a good patch. The band aid is to protect the sensitive wound from bumps and bruises. It is also to stop you from picking at the newly formed patch of clotted blood.
In a few days, the clot of blood becomes crusty and brown. It may pull at the skin and make you want to scratch and pull it off. But this would only break the little blood vessels again and you would have to wait for a new scab to form. When the wound is healed, the scab falls off by itself.
The scab material is formed by wonderful chemicals in the blood. When the blood is safely shut inside the blood vessels, these chemicals are runny liquids. They form clots and scabs only when the blood seeps out of a wound. A powerful microscope shows that a scab is made from a mesh of tough threads that look somewhat like layers and layers of chicken wire.