7.4 - 7.5 - The menstrual cycle
Follow changes in the ovaries and uterus through a menstrual cycle. Learn the common-tier roles of oestrogen and progesterone, then explore the four-hormone control sequence at Higher tier.
7.4 - One cycle from period to period
The menstrual cycle coordinates changes in two organs: an egg develops and may be released from an ovary, while the inner lining of the uterus changes so that it could support an embryo. The cycle is counted from the first day of one period to the day before the next.
Menstrual cycle
The repeating sequence of changes in the ovaries and uterus that includes menstruation, preparation for ovulation, ovulation and preparation of the uterus lining for a possible pregnancy.
A 28-day cycle is a useful model for learning the sequence, but it is not a timetable that every person follows. Cycle length varies, and ovulation is not always on day 14.
- Menstruation begins the cycle. Day 1 is the first day of bleeding. Part of the uterus lining breaks down and leaves the body; oestrogen and progesterone are low.
- An egg develops and the lining is repaired. In the first part of the cycle, a follicle containing an egg develops in an ovary. The ovary produces increasing oestrogen, which repairs and thickens the uterus lining after menstruation.
- Ovulation releases an egg. At about the middle of the idealised cycle, a mature egg is released from an ovary. This release is ovulation; it is not menstruation.
- The lining is maintained, then the cycle resets if there is no pregnancy. After ovulation, the ovary produces more progesterone. Progesterone maintains the thick uterus lining. If pregnancy does not occur, progesterone and oestrogen levels fall, the lining is no longer maintained, and the next menstruation begins.
Oestrogen repairs and thickens the uterus lining before ovulation; progesterone maintains it after ovulation. A fall in both hormones leads to menstruation.
7.5 - Higher: four-hormone control
Higher-only (7.5). The cycle is controlled by interactions among two pituitary hormones, FSH and LH, and two ovarian hormones, oestrogen and progesterone. Hormones travel in the blood, but only target tissues with the appropriate receptors respond.
| Hormone | Main source in this model | Main action in the cycle |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Stimulates a follicle and its egg to mature; also stimulates the ovary to produce oestrogen |
| Oestrogen | Ovary, mainly the developing follicle | Repairs and thickens the uterus lining; inhibits FSH; a high level stimulates a surge of LH |
| LH (luteinising hormone) | Pituitary gland | Its surge triggers ovulation and formation of the corpus luteum from the emptied follicle; this produces progesterone |
| Progesterone | Ovary after ovulation | Maintains the uterus lining and inhibits FSH and LH |
These actions form one causal sequence:
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