system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If your period is just blood and uterine matter that your body has been storing up for a month expelling from your body, then why do some women become anemic?

I'm currently on my period and it's been a rough one. I've been very weak and having hot flashes and craving red meat like crazy. I have a tendency of becoming anemic during my period which leads me to this question.

I know periods are from your uterus getting ready fro pregnancy and expelling it when there is no fertilized egg. What I don't understand is if it's a month long process, then why does it make some women anemic? Are we losing NEW blood?
gpt
Yes, you are losing it only then and yes, it's new.

Your uterus does not fill with old blood all month, it builds an increasingly big layer of tissue on its walls.

When you are not pregnant, a lack of hormones causes the vessels nourishing the tissue to close, the tissue dies.

The muscular part of the uterus then starts contracting and the vessels close and open periodically to get rid of the old tissue. That's when you bleed.

Normally the amount of blood lost during menstruation is quite small, about 50-120 ml. As a reference: when donating blood you give 500 ml.

If you loose more than that or your blood levels are already low maybe that could make you anemic.