Eli5 : Why does it burn when water touches an open wound?
In: 5
When you are injured, your cells send out a bunch of chemical signals that cause inflammation. Some of these signals dilate your blood vessels so you get more resources to fix the injury. Other signals call more immune cells over to protect against infection.
One of the signals sent out makes all of the pain-sensing nerve cells more sensitive. For example, if you push on your skin a little bit it probably doesn’t hurt. If you push a bit harder it can start to hurt a little. The nerves that sense pain caused by pressure aren’t sensitive enough to go off from just a little bit of pressure – which is good. If you get injured, you will notice that just a little bit of pressure in that area causes pain, because those nerves are made more sensitive.
This is on purpose, so that you will protect an injured area, giving it time to heal. Pain is the body’s way of warning you and protecting you.
Because all of your pain-sensing nerves are more sensitive, water will set a lot of them off when it normally wouldn’t. You also have nerves that are buried in your skin so that they *only* go off when a cut is deep enough to expose them, which the water will hit and set off.
You’ll notice that cold or hot water probably hurts more, because you also have nerves that sense dangerous cold or heat, which are made more sensitive.
So, you’ve got a lot of stimuli affecting a very sensitive area.
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