Organism level systems Study Guide

The menstrual cycle, typically lasting about 28 days, is regulated by a complex interplay of four key hormones: Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), oestrogen, and progesterone. The cycle begins with the release of FSH from the pituitary gland. FSH stimulates the development of follicles in the ovary and also promotes the production of oestrogen by these developing follicles. As oestrogen levels rise, they initially inhibit FSH production (negative feedback), but then, as they reach a high threshold, they trigger a surge in LH release from the pituitary gland. This LH surge is crucial for ovulation, causing the mature follicle to burst and release an egg (usually around day 14). After ovulation, the ruptured follicle transforms into the corpus luteum, which then produces progesterone and some oestrogen. Progesterone is vital for maintaining the uterine lining, preparing it for a potential pregnancy, and it also inhibits the release of FSH and LH (negative feedback). If fertilisation and implantation do not occur, the corpus luteum degenerates, leading to a sharp drop in progesterone and oestrogen levels. This decline in hormone levels causes the uterine lining to break down and shed, resulting in menstruation. The low levels of oestrogen and progesterone then remove the inhibition on FSH production, allowing FSH to rise again, restarting the cycle.

The human reproductive system is elegantly controlled by hormones, chemical messengers produced by endocrine glands. In females, the menstrual cycle is a monthly process regulated by a complex interplay of four key hormones: Follicle-Stimulating Hormone (FSH), Luteinising Hormone (LH), oestrogen, and progesterone. This cycle prepares the uterus for a possible pregnancy. It typically lasts about 28 days, though this can vary for individuals.

The cycle begins with the pituitary gland releasing FSH. FSH stimulates the development of follicles in the ovaries, each containing an egg, and also encourages the production of oestrogen by the ripening follicle. As oestrogen levels rise, they initially inhibit FSH production but then, at a certain threshold, trigger a surge in LH from the pituitary gland. This LH surge causes ovulation – the release of a mature egg from the ovary – usually around day 14 of the cycle.

After ovulation, the empty follicle transforms into the corpus luteum, which starts producing progesterone and some oestrogen. Progesterone is crucial for maintaining the uterine lining, making it thick and rich in blood vessels, ready for implantation of a fertilised egg. High levels of progesterone and oestrogen inhibit the release of FSH and LH, preventing further follicle development and ovulation.

If fertilisation and implantation do not occur, the corpus luteum degenerates, leading to a drop in progesterone and oestrogen levels. This decline causes the uterine lining to break down and shed, resulting in menstruation (a period). The low levels of oestrogen and progesterone then allow FSH levels to rise again, starting a new cycle. If pregnancy occurs, progesterone levels remain high, sustained initially by the corpus luteum and later by the placenta, to maintain the uterine lining and prevent further menstrual cycles during pregnancy.