Why does a knock to the head cause swelling?”
The skin on the scalp is very thin compared to the rest of the body, and does not have a large fatty layer. The blood vessels run between the skin and the skull, instead of in the fatty tissue here. When a blow to the head breaks these blood vessels, they leak between the skin and skull, instead of in to the different layers of the skin. They bleed so much that the lump is raised from the pressure of all of the fluid. You can’t normally see the bruise because all of the blood is beneath all of the layers of the skin, instead of the blood leaking from within the skin into different layers. It is not impossible for a bruise to form on the scalp, it is just rare. You are correct that other parts of the body swell in a different manner. This is because of the different arrangement of the fatty layers of the skin and the blood vessels. I hope this helps.
