system
Given the following post, respond as if you're commenting on Reddit:
human
Played guitar for the first time in a while which resulted in a blister on my fingertip. Almost immediately, there was fluid in it. Where does that fluid come from?
gpt
It's basically blood plasma--the liquid part of blood without any red blood cells, which is water with sugars, proteins, and some other things dissolved in it. Pretty much all of the cells in your body are floating in a bath of this stuff all the time, which leaks out from the capillaries in your body. This is how cells get and exchange gas and nutrients. 

Essentially the deeper layers of your skin got damaged from the pressure from the strings. When you get tissue damage, the small blood vessels around the area start opening up their walls a bit to let more of the fluid out along with a few white blood cells to clean up anything that got into the wound. Since the top layer of your skin didn't get damaged, the fluid just kinda collects underneath it as it leaks out. The reason it's not filled with blood is because there wasn't enough damage to the tissue to break any blood vessels, which would lead to red blood cells getting out.