Menstrual flow refers to the blood and tissue that is shed from a woman's uterus during her menstrual cycle. It occurs when the lining of the uterus, which has built up in preparation for a potential pregnancy, breaks down and is expelled through the vagina. This process typically happens on a monthly basis in women who are not pregnant. The flow can vary in amount from woman to woman, as well as in color and consistency. It usually lasts for about 3-7 days per cycle but may vary depending on individual factors such as hormonal balance or stress levels.