Where does menstrual blood flow from?

Menstruation is mainly from the uterine endometrium, endometrium contains glands and blood vessels, under the action of estrogen will proliferate, estrogen and progesterone will suddenly drop before the arrival of menstruation, endometrium will appear exfoliation bleeding, thus producing menstrual blood. Under normal circumstances, menstrual volume between 20-80ml, menstrual period 3-7 days.