4.6.3Additional lessons
The Menstrual Cycle — IB Biology SL Revision Notes & Topical Questions
5 min read 5 parts Lesson 118 of 142
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Phases of the Cycle
An average cycle lasts about 28 days, though cycle length varies between individuals.
1
Day 1 is the first day of menstruation, when the endometrium from the last cycle breaks down and is shed with some blood.
2
The pituitary gland releases FSH, which stimulates several follicles in the ovary to develop; each follicle contains a maturing egg cell.
3
Growing follicles secrete oestrogen, which rebuilds the endometrium and makes it thicker with a good blood supply.
4
A surge of LH triggers ovulation at about day 14, when a mature follicle releases its egg into the oviduct.
5
The empty follicle becomes the corpus luteum, which secretes progesterone, maintaining the thick endometrium ready for an embryo.
6
If no embryo implants, the corpus luteum degenerates, hormone levels fall and the cycle restarts with menstruation.
Section 2 of 5The Four Hormones
FSH
- FSH Follicle-stimulating hormone, from the pituitary, stimulates follicle development in the ovary.
LH
- LH Luteinising hormone, from the pituitary, triggers ovulation and the formation of the corpus luteum.
Oestrogen
- Oestrogen Secreted by growing follicles, it repairs and thickens the endometrium and affects pituitary secretion.
Progesterone
- Progesterone Secreted by the corpus luteum, it maintains the endometrium and inhibits FSH and LH release.
| Hormone | Source | Main effect |
|---|---|---|
| FSH | Pituitary | Follicle growth |
| LH | Pituitary | Ovulation, corpus luteum |
| Oestrogen | Follicle | Endometrium repair |
| Progesterone | Corpus luteum | Maintains endometrium |
Section 3 of 5Feedback Control
Feedback Control
Early negative feedback
- Early negative feedback Rising oestrogen early in the cycle at moderate levels inhibits FSH secretion,
- which prevents too many follicles from maturing.
Positive feedback
- Positive feedback When oestrogen reaches a high peak just before ovulation it stimulates pituitary release of LH and FSH, causing the LH surge.
After ovulation
- After ovulation High progesterone and oestrogen from the corpus luteum inhibit FSH and LH, so no new follicles begin to develop.
Cycle reset
- Cycle reset When the corpus luteum fails, progesterone falls, inhibition stops and FSH can rise to start new follicle growth.
Section 4 of 5Reading Hormone Graphs
Oestrogen peak
- Oestrogen peak The first peak occurs shortly before ovulation, followed by a smaller second rise during the luteal phase.
LH surge
- LH surge A sharp peak at about the middle of the cycle marks the trigger for ovulation.
Progesterone
- Progesterone Levels are low until after ovulation, then rise to a peak and fall before menstruation.
Endometrium thickness
- Endometrium thickness It is thinnest around menstruation and thickest in the days before the next one if no embryo implants.
Section 5 of 5Exam Checklist & Tips
4.6.3 – Key Points for the Exam
- Ovulation Release of an egg from a mature follicle in the ovary.
- Follicle Fluid-filled structure in the ovary that contains a developing egg.
- Corpus luteum Structure formed from the empty follicle that secretes progesterone.
- Endometrium The lining of the uterus that thickens to receive an embryo.
- Negative feedback Process in which a change causes responses that reduce that change.
Common Mistakes
- Saying that FSH and LH are made in the ovary; they come from the pituitary gland.
- Stating ovulation occurs at the end of menstruation; it is about the middle of the cycle.
- Claiming progesterone triggers ovulation; the LH surge does.
- Confusing the corpus luteum with the follicle; the corpus luteum forms after ovulation.
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