system
Given the following post, respond as if you're commenting on Reddit:
human
During a woman's period, why is blood used as the medium to flush the system out rather than using a different fluid?

It seems strange that the system would use the most precious fluid in the body to flush out the system during a menstruation cycle. Wouldn't the production of a mucus be better?
gpt
It's not actually blood.

The women sheds the wall of the uterus which has been thickening for the previous 15-20ish days. It just so happens that the uteren wall has to be full of blood vessels in the case that a fertalized egg attaches to it. 

Lots of blood vessels means it will have a lot of blood in it, which gives it the characteristic red colour.