system
Given the following post, respond as if you're commenting on Reddit:
human
How is it that the human body, which is full of bacteria and germs, can produce sterile fluids? For example, I've heard that the fluid inside blisters is sterile and so is urine.

Edit: Thanks for the fascinating discussion. I'm learning a lot. Still reading.
gpt
There aren't bacteria **inside** a healthy human body. There are bacteria in the lumen of the digestive tract, which is part of the outside world. 

Think of the digestive tract as a tunnel running through the body from the mouth to the anus. The inside (lumen) of that tunnel is continuous with the outside world, so is not actually inside the tissues of the body. Similarly, your skin is continuous with the lining of your digestive tract, all of which forms a barrier that protects your body from invaders (bacteria, etc). If bacteria breach that barrier, your immune system responds to destroy the invaders. 

That's why most body tissues and fluids (including blood, urine, cerebrospinal fluid, and the blister fluid you refer to) are normally sterile. They've never been exposed to the outside world and its menagerie of bacteria.

(Edit: words)