Menstruation occurs because the inner lining of the uterus (called the endometrium) has undergone changes that prepare for pregnancy. If there is a fertilized egg, the uterus will be able to provide a hospitable and nurturing environment. If a woman doesn’t become pregnant, the inner lining is shed, in a discharge that includes cells, fluid, and blood; we call this menstruation.
Fun fact #1: Menstruation is a rarity, perhaps an oddity, in nature. Most animals don’t prepare a lining in advance just in case pregnancy occurs. They have hormonal cycles, which are called estrus cycles, but not menstrual periods. Most primates (monkeys, apes, and us) menstruate (Nelson, page 262).
Fun fact #2: The endometrium is shed in layers during menstruation (Voda, page 62). That is, although cells are dying, blood vessels are leaking, and other changes are occurring, this breakdown occurs in the form of shedding of layers, not in a random cataclysm.
Fun fact #3: Most of the endometrial material that is eliminated is resorbed, not menstruated (Voda, page 62). That is, your body typically clears away material that is no longer useful or is waste. Suppose, for example, you bang your finger hammering a nail and get a bruise. Over time, the black-and-blue area at your injury, with its injured cells, blood, and other material, returns to normal as your body clears away the debris. Similarly, according to Voda, most of the endometrial material is resorbed; menstrual flow is a minority of the material being shed.
Fun fact #4: Why do women (or monkeys and apes, for that matter) menstruate?
Answer: There are theories, but no one knows.
Nelson, Randy. An Introduction to Behavioral Endocrinology. Sinauer Associates.
Voda, Ann. Menopause, me and you. Haworth Press.