Eli5: Why do we get a bump on our head when we bang it against something but nowhere else on the body?
In: Biology
I don’t know a ton about this, so I could be wrong. But my take on it is this: our heads have a LOT of blood vessels. Banging your head against something can break many of these blood vessels, causing a lot of blood to be pushed out and collect under the skin. The blood pools and the area swells up into the bump you are describing. This is called a scalp hematoma (which is often harmless). Hematomas can happen elsewhere on the body, and on your organs (which can be serious!) but they tend to happen with larger blood vessels. When you break smaller blood vessels, it causes a bruise. It’s less blood than a hematoma, so it doesn’t cause a big bump. So when you bang your arm against something it’s way more likely that you’ll get a bruise than a hematoma.
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