Endometriosis. Artwork of the organs of the female lower abdomen and sites (purple) of endometriosis. Endometriosis is where parts of the uterine lining (endometrium) enter the abdominal cavity instead of exiting the uterus through the vagina as during a normal menstrual cycle. The endometrial tissue attaches to other abdominal organs and forms cysts that bleed during menstruation and can cause pain, inflammation and scarring. The front of the body, and the pubic bone (brown) is at left. From left to right, from the pubic bone, all sectioned, are: the bladder, the vagina and the rectum. The uterus is above the bladder and has two Fallopian tubes, which each end near an ovary.