Final answer:
The menstrual cycle involves the ovarian cycle (follicular phase, ovulation, luteal phase) and the uterine cycle (menstruation, proliferative phase, secretory phase). It typically spans 28 days but can vary between women and from month to month, with menstruation lasting from 3 to 7 days.
Step-by-step explanation:
The menstrual cycle is a sequence of natural changes in the female reproductive system that is crucial for egg production and preparing the uterus for pregnancy. The cycle is split into the ovarian cycle and the uterine cycle, each with distinct phases. The ovarian cycle includes the follicular phase, ovulation, and the luteal phase. During the follicular phase, a follicle matures due to increased levels of Follicle Stimulating Hormone (FSH); ovulation then occurs when estrogen levels spike and Luteinizing Hormone (LH) surges, causing the egg to be released. If pregnancy does not take place, the luteal phase ends with the corpus luteum degrading, leading to a decrease in progesterone and the onset of menstruation.
The uterine cycle, which runs concurrently to the ovarian cycle, is made up of menstruation, the proliferative phase, and the secretory phase. Menstruation, which signals the start of the cycle, involves the shedding of the endometrial lining and usually occurs from days 1 to 5. Post-menstruation, the endometrium rebuilds during the proliferative phase until ovulation and finally, during the secretory phase, prepares to possibly receive an embryo by secreting nutrients and undergoing structural changes.
While the average cycle length is 28 days, it can normally vary between 21 to 32 days, with menstruation typically lasting 3 to 7 days. These variations may occur from one woman to another as well as from one month to another within the same individual.