system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do they tell you not to have a hot bath on the day of a blood donation?

I was reading the blood donation website and it says 'Please do not have a hot bath on the day of your donation'. I just wondered why that was.
gpt
A hot bath tends to make you flush, moving blood from the interior to the exterior of your body. Your blood volume is lower from the donation and it might not have had time to adapt, so you might faint.