4.7.3Additional lessons
The Menstrual Cycle — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 57 of 101
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 5Events of a 28-day Cycle
The cycle is a series of events in the ovary and uterus coordinated by hormones from the pituitary and ovary.
1
Days 1 to 5: the uterine lining breaks down and is lost as menstruation, while FSH rises and stimulates follicles to grow.
2
Days 6 to 13: the developing follicle secretes oestradiol, which repairs the endometrium and at first inhibits FSH and LH.
3
Around day 12 to 13: high oestradiol acts by positive feedback on the pituitary, causing a surge in LH and some FSH.
4
Around day 14: the LH surge triggers ovulation, and the ruptured follicle becomes the corpus luteum.
5
Days 15 to 28: the corpus luteum secretes progesterone and oestradiol, maintaining the lining; if there is no pregnancy it degenerates.
Section 2 of 5Hormones and Their Roles
FSH
- FSH Secreted by the anterior pituitary, it stimulates follicle growth in the ovary.
LH
- LH Secreted by the anterior pituitary, the mid-cycle surge triggers ovulation and the formation of the corpus luteum.
Oestradiol
- Oestradiol Secreted by the follicle, it stimulates repair and thickening of the endometrium and controls FSH and LH release.
Progesterone
- Progesterone Secreted by the corpus luteum, it maintains the thickened endometrium and inhibits FSH and LH.
| Hormone | Source | Target and effect |
|---|---|---|
| FSH | Anterior pituitary | Ovary: follicle development |
| LH | Anterior pituitary | Ovary: ovulation, corpus luteum |
| Oestradiol | Follicle, corpus luteum | Uterus lining, pituitary |
| Progesterone | Corpus luteum | Uterus lining, pituitary |
Section 3 of 5Feedback Control
Negative feedback
- Negative feedback Early rising oestradiol and later progesterone inhibit FSH and LH, preventing more follicles from developing.
Positive feedback
- Positive feedback Once oestradiol is high for enough time, it stimulates the LH surge instead of inhibiting it.
Falling levels
- Falling levels When the corpus luteum degenerates, progesterone falls, the lining cannot be maintained and menstruation begins.
Pregnancy
- Pregnancy If an embryo implants, hormonal signals stop the corpus luteum degenerating, so the lining stays.
Section 4 of 5Reading Hormone Graphs
Peak order
- Peak order Oestradiol peaks first, then the LH surge, then ovulation, then progesterone rises in the second half.
Ovulation timing
- Ovulation timing Ovulation occurs within roughly a day of the LH peak.
Cycle length
- Cycle length The cycle length varies between women, so day numbers are averages.
Section 5 of 5Exam Checklist & Tips
4.7.3 – Key Points for the Exam
- Ovulation Release of a secondary oocyte from the ovary.
- Corpus luteum The structure formed from the follicle after ovulation, secreting progesterone.
- Menstruation Loss of the uterine lining through the vagina at the start of the cycle.
- Positive feedback A change causing further change in the same direction.
Common Mistakes
- Saying that progesterone triggers ovulation; the LH surge does.
- Writing that FSH comes from the ovary; it is produced by the pituitary.
- Thinking that menstruation happens at ovulation.
- Describing all feedback in the cycle as negative.
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