Eli5: Why do we get a bump on our head when we bang it against something but nowhere else on the body?
In: Biology
First, most other places in the body are soft and that softness elsewhere lessens the impact of an injury at that site.
Second, the swelling on a scalp is forced out from the surface due to the hardness of the bone. Like a ball sitting on a table rather than a ball floating in a pond.
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.
Easiest way to put it is there’s no padding/fat. You can get bumps on other parts but it would take a more severe hit. When you get injured blood rushes towards it. Think of it like plugging a dam. Now picture the dam is your fat. Now if you take the dam away and it starts to pour rain aka a head injury the blood still rushes there but now it’s trying to act as a damn. By doing this the body tries to go into a survival mode and it typically adds a calcium like layer which is usually the bump on the head.
You can get bumps (I think the medical term is contusion) anywhere, they’re just especially noticable on the head where there’s less soft tissue than other areas.
I get bumps all the time whenever I bang my shin against the table at 3 am. Also get bumps if I get hit in certain places on the forearm. Maybe it’s because most of us look at our faces more often than anywhere else, so we notice these things more?
You do get bumps all over the body depending upon how serious the “bang.” Anywhere there is bone underneath like the shin or elbow especially. Even a soft tissue injury can form a bump from inflammation and bruising.
Other parts of the body do swell if we hit them. Pretty sure our head is somewhat different because of the cranium.
Basically, lack of padding (muscle or fat) + proximity to bone = swelling.
Other places you could develop swelling would be shins, elbows, shoulder (where your arm bone connects to the long shoulder bone), and, as anyone who fights or rages on inanimate objects), your knuckles.
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