Where dose period blood come from?

Uterus: In detail, your uteral lining called the endometrium becomes engorged with bood due to effects of estrogen. When levels of estrogen peaks and dips, the blood gorged endometrium is no long maintained and thus menses occur. There are other hormones at play including progesterone, lh, fsh all of which are vital to ovulation and menstural cycle. Hope this helps. Otherwise you can look up further.