Visually, how does menstruation work in the uterus?
Take a look at the visual linked here. Basically what's happening during menstruation is that the substances from inside the uterus (inner part of the organ) shed. This is because the body prepared for carrying a baby but if no fertilization occurs, there is no need for all the substances that have been built up inside. Menstrual flow includes blood, mucus, and other substances.
Menstruation is just one part of the overall cycle when the lining of the uterus (endometrium) is shed. This occurs throughout a menstruator's reproductive life. With each monthly cycle, the endometrium prepares itself to nourish a fetus. Increased levels of estrogen and progesterone help thicken its walls. If fertilization does not occur, the endometrium, along with blood and mucus from the vagina and cervix make up the menstrual flow that leaves the body through the vagina during the period.
