What menstrual flow contains
The uterine lining, or endometrium, is a layer of tissue that changes over the course of a menstrual cycle. When it sheds, it usually breaks apart rather than coming away as one intact sheet. The resulting flow contains blood and fragments of endometrial tissue, mixed with mucus and fluid from the reproductive tract.
That mixture is why menstrual flow can look and feel different from bleeding from a cut. Its color and texture may vary as blood and tissue leave the uterus at different rates; small clots can form when blood pools before passing out.
How the shedding begins
Hormones guide the cycle. After ovulation, progesterone helps maintain the built-up lining. If pregnancy does not occur, progesterone levels fall, prompting the lining’s surface layer to break down. The uterus then contracts, helping move the material through the cervix and out of the vagina. Substances called prostaglandins contribute to those contractions, which is one reason they can also cause menstrual cramps.
The lining is not simply discarded and left depleted: the basal layer beneath the part that sheds remains, and cells there help rebuild the endometrium for a later cycle. Each period is therefore both an exit and a reset—the visible flow carries away tissue from one cycle while the uterus begins preparing its lining again.
