Where does menstrual blood come from?

The blood shed during menstruation originates from the endometrium—the inner mucosal lining of the uterus. During the proliferative and secretory phases of the menstrual cycle, rising estrogen and progesterone levels stimulate endometrial thickening and glandular development in preparation for potential embryo implantation. If fertilization and implantation do not occur, circulating progesterone (and subsequently estrogen) levels decline sharply due to corpus luteum regression. This hormonal withdrawal triggers vasoconstriction of the spiral arterioles supplying the functional layer of the endometrium, leading to ischemia, tissue necrosis, and subsequent shedding of the superficial endometrial tissue—primarily the functionalis layer—along with blood, mucus, and cellular debris. The basal layer of the endometrium remains intact and serves as the regenerative source for the next cycle. Thus, menstrual blood is not venous or arterial blood per se, but rather a mixture of arterial and venous blood from disrupted endometrial vessels, along with exfoliated endometrial cells and glandular secretions.