The main source of menstrual blood during menstruation is:

Menstrual bleeding is from both systems, but aerial bleeding is appreciably greater than venous. Endometrial bleeding appears to follow rupture of an aeriole of a coiled aery, with consequent hematoma formation. With a hematoma, the superficial endometrium is distended and ruptures. Hemorrhage stops with aeriolar constriction. Ref: Cunningham F.G., Leveno K.J., Bloom S.L., Hauth J.C., Rouse D.J., Spong C.Y. (2010). Chapter 3. Implantation, Embryogenesis, and Placental Development. In F.G. Cunningham, K.J. Leveno, S.L. Bloom, J.C. Hauth, D.J. Rouse, C.Y. Spong (Eds), Williams Obstetrics, 23e.