Where does menstrual blood originate from? Helpful to learn for both women and fathers-to-be☝🏻 Actually, it's more correct to call it menstrual fluid. Since, besides the blood itself, it includes the mucous secret of the cervical glands, vaginal glands and endometrial tissue. This fluid is secreted just on account of the latter element in this list. During menstruation the functional layer of the endometrium (the mucous membrane of the uterus) is detached. Blood vessels get involved in this process, where the blood oozes out. An average and conventionally normal volume of menstrual fluid is considered to be 40-80 ml.