Final answer:
The menstrual phase begins due to a decrease in progesterone levels, which results in the shedding of the endometrial lining of the uterus. This phase coincides with the follicular phase of the ovarian cycle, where hormone levels are low as the corpus luteum degrades.
Step-by-step explanation:
The menstrual phase of the female cycle commences due to a decline in the levels of certain hormones, primarily progesterone. The menses phase, or menstruation, occurs when the endometrial lining of the uterus is shed. This phase marks the beginning of the menstrual cycle and typically lasts between 2 to 7 days.
It coincides with the early days of the follicular phase of the ovarian cycle, when the levels of progesterone, follicle-stimulating hormone (FSH), and luteinizing hormone (LH) are low due to the degradation of the corpus luteum at the end of the luteal phase. As progesterone levels fall, the constriction of endometrial arteries leads to the breakdown of the endometrium, which is then expelled from the body through the vagina.
The menstrual cycle is also influenced by the ovarian cycle, which includes follicular growth, ovulation, and if no pregnancy occurs, the degeneration of the follicle. These cyclical events are regulated by hormones from the hypothalamus, pituitary gland, and ovaries.