In the female reproductive cycle menstrual flow occurs when concentrations of?

In the human female reproductive cycle, menstrual flow occurs when the levels of the hormone progesterone decrease sharply. Progesterone is primarily produced by the corpus luteum, a temporary structure formed in the ovary after ovulation. Its primary role is to maintain the uterine lining (endometrium) in a thickened and supportive state, suitable for implantation of a fertilized egg.

During the luteal phase of the menstrual cycle, progesterone levels rise and contribute to the thickening and vascularization of the endometrium in preparation for possible embryo implantation. If implantation does not occur, the corpus luteum starts to regress, leading to a significant drop in progesterone levels.

The sudden drop in progesterone causes the uterine lining to break down and shed, leading to menstrual flow. Menstrual flow typically lasts for several days and marks the beginning of a new menstrual cycle.

It's worth noting that the menstrual cycle is regulated by a complex interplay of various hormones, including estrogen and progesterone, produced by the ovaries and the hypothalamus-pituitary-gonadal axis in the brain.