Final answer:
The menstrual phase of the uterine cycle is caused by the degeneration of the corpus luteum, a drop in progesterone levels, and the destruction of the endometrium. These factors lead to the shedding of the endometrial tissue and the start of the next menstrual cycle.
Step-by-step explanation:
The menstrual phase of the uterine cycle is caused by all of these. When no fertilization occurs, the corpus luteum degenerates, leading to a drop in the blood levels of estrogen and progesterone. This hormonal decrease causes the endometrium (the lining of the uterus) to break down, initiating the menstrual cycle.
This includes the degeneration of the corpus luteum, a drop in the blood levels of progesterone, and alternating vasoconstriction and vasodilation of the spiral arteries destroying the functional layer of the endometrium. A non-pregnant woman experiences the breakdown of the endometrium and the shedding of the endometrial tissue, resulting in menses or the start of the next menstrual cycle.