What causes swelling?

When you bump your head, what exactly makes it swell up into a big bump like that?

Why does any other part of your body swell up so easily? I’ve bumped my bicep and knee, among other parts, into things but they’ve never swelled up.

Inflammation. Inflammation is redness, swelling, heat, and pain.

The mechanism of inflammation is more involved:

In simplified terms, the swelling is caused because the blood leaks out of your arteries and into the surrounding area. It leaks out for two reasons.

Firstly you’ve physically damaged the arteries. If you run over your garden hose when it’s lying on concrete it springs a leak and the fluid runs out. Same thing happens if you crush and artery between the underlying bone and a hard object.

Secondly you’ve damaged the surrounding tissue, and that causes the tissue to release hormones that make the arteries leaky. The idea behind that is that as blood leaks out of the arteries it carries white blood cells that both stop infection and also remove any damaged material. So they clean up the damage and protect the area while it’s being cleaned up.
(This being the SDMB some peant is going to come up with a lot of confusing details about why this isn’t correct, but given the nature of the question I think it’s a pretty safe bet that discussing “lymph” and “arterioles” and “histamines” is just going to confuse the issue.)

As for why other areas don’t swell. It’s because the bone of the skull lies right under the skin.

Few other areas have rigid bone lying that close to the surface. That means that you can’t crush the arteries against the bone and rupture them. The bone of the knee cap may be near the surface, but the kneecap itself isn’t rigid, it moves, thus preventing damage.

The only other places where you have bone against the skin like that are the shins and the forearms. However those areas are damaged so much in normal life that the blood vessels have become somewhat hardened and more difficult to rupture. People who routinely headbutt hard objects don’t experience swelling on their heads either, for the same reason.

Having bone right under the skin also means that the swelling has no way to go but out. If you damage your chest, for example, the swelling is forced inwards as much as outwards, and so appears much smaller. But in the case of the skull the swelling can only protrude out, making any swelling appear huge.

Blake, it’s certainly possible to explain a process like this using layman’s terms without being incorrect on so many accounts.

Also known as, “No brain, no pain.”

Could you expand, USCD? The only major thing I would change in Blake’s description is “arteries” to the more generic “blood vessels”.

Here’s wiki’s take on the “exudation” aspect of inflammation, when fluid begins to accumulate near the injury site: