system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When giving an IV or a getting a shot, how does the nurse/doctor know they're in the vein and haven't pushed the needle out of the back of the vein?

Holy shit, lot of information to wake up to.  Thanks everyone!
gpt
It's really a two part answer:

1. Once you're in the vein, you will get a "flashback" which is blood that flows out of the vein into the catheter.

2. Then the  person flushes the IV with saline.  If the tissue around the area "bubbles" or swells, then you are no longer in the vein (you either didn't go enough in or you punctured through).