Last updated: Sun, Jun 18, 2017
After about 15 seconds the white line will disappear as the blood vessels relax. The white line will be replaced by a red area along the wound. Nerve cells and other cells in the skin that were injured by the scratch released a variety of chemicals into the injured area. It's believed that when some of the terminals of a sensory neuron fire, the action potential spreads to the other terminals of the same neuron and causes them to release inflammatory chemicals as well. This expands the area of the inflammatory reaction.
One of the effects of these chemicals is to dilate the vessels that formerly were in spasm, to the point that the circulation near the surface increases. The area appears red and feels warm. The initial sharp pain is now replaced by a feeling of soreness or itchiness, depending on how severe the injury is.