The uterine cycle begins with the menstrual phase, which is considered day one of the cycle and typically lasts about five days. This phase is characterized by the degeneration and shedding of the stratum functionalis, the functional layer of the endometrium.
When fertilization does not occur, the corpus luteum deteriorates, causing a significant drop in the levels of estrogen and progesterone in the body. This hormonal decrease triggers the release of prostaglandins, which cause the uterine spiral arterioles, small branches of arteries, to constrict.
This constriction results in the stratum functionalis being deprived of oxygen and nutrients. The deprivation leads to the breakdown of secretory glands and other tissues within the stratum functionalis. As deprivation continues, the arterial walls weaken and rupture, allowing blood to seep into the connective tissues of the functional layer.
This process culminates in menstruation, during which blood, tissue fluid, mucus, and epithelial cells are expelled through the vagina. The endometrium thins to approximately 2–5 mm, leaving only the stratum basalis intact to regenerate the next cycle's functional layer.
While many experience menstruation without significant discomfort, some women suffer from dysmenorrhea or painful menstruation. This pain can result from myometrial contractions ("cramps"), uterine inflammation, or conditions affecting adjacent pelvic structures.
The menstrual cycle starts with the menstrual phase, with its first day considered Day 1 of the cycle.
This phase usually lasts for 1 to 5 days and is characterized by the degeneration and shedding of the endometrial functional layer, or stratum functionalis.
The lack of fertilization leads to degeneration of the corpus luteum, decreasing the circulating levels of estrogen and progesterone.
Their decreased levels trigger the release of prostaglandins, which cause the uterine spiral arterioles to constrict.
Deprived of oxygen, the secretory glands and other tissues in the stratum functionalis start to break down.
The weakened arterial walls eventually rupture, disrupting the tissue organization of the stratum functionalis.
Blood cells and deteriorating tissues are then shed through the vagina in a process known as menstruation.
The resulting vaginal discharge consists of a mixture of blood, tissue fluid, mucus, and epithelial cells shed from the endometrium.
As a result, the endometrium thins to approximately 2 to 5 mm, with only the stratum basalis layer remaining.
繁體中文
