A mature corpus luteum produces high levels of progesterone; it’s a glandular structure which is formed briefly by enlarged follicular cells at ovulation. When the corpus luteum ceases to function, the concentrations of estrogen and progesterone (hormones) decline rapidly, and in response, blood vessels in the endometrium (uterine lining) become constricted and soon disintegrate and slough away. Atmycontentbreak the same time, blood escapes from damaged capillaries, creating a flow of blood and cellular debris that passes through the vagina as the menstrual flow. The beginning of the menstrual flow marks the end of a menstrual cycle and the beginning of a new cycle.