system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do our veins handle extra liquid from shots or IV

I was in surgery earlier and remember when the nurse put injected anesthesia and something else through the IV and I could feel the liquid push itself into my vein and it felt like my vein in my hand to elbow would explode. 

That got me thinking, how does my body intake so much liquid like the full IV bag without exploding or peeing on the operating table  while I'm knocked out.
gpt
Veins and arteries pump liquid through your veins really quickly. It is constantly moving. So, extra fluid in the veins from an IV is no different from drinking a huge amount of liquid for it to also be absorbed. Once the nutrients go where they need to go, the kidneys filter out the excess. And you would absolutely urinate on the operating table, but you most likely had a catheter in that was inserted and removed while you were knocked out. 

The sensation of feeling the injection into your veins isn’t really about volume, but more that what was injected was an irritant. That’s why it is diluted with neutral IV fluids from the full bag.