Hormonal Coordination (B5.3)
Endocrine system
Hormones are chemical messengers secreted by endocrine glands directly into the blood. They travel to target organs where they bind to specific receptors. Effects are slower but longer-lasting than nervous system signals.
| Gland | Hormone | Function |
|---|---|---|
| Pituitary | FSH, LH, ADH, growth hormone | "Master gland" — controls other glands |
| Thyroid | Thyroxine | Controls metabolic rate |
| Adrenal | Adrenaline | Fight-or-flight response |
| Pancreas | Insulin, glucagon | Blood glucose regulation |
| Ovaries | Oestrogen, progesterone | Female sexual development, menstrual cycle |
| Testes | Testosterone | Male sexual development |
Menstrual cycle
Controlled by four hormones and involves negative and positive feedback:
- FSH (follicle-stimulating hormone — pituitary): stimulates follicle development in ovary; stimulates oestrogen production.
- Oestrogen (ovary): repairs/thickens uterus lining; at high levels triggers LH surge (positive feedback).
- LH (luteinising hormone — pituitary): triggers ovulation (~day 14); stimulates corpus luteum.
- Progesterone (corpus luteum): maintains uterus lining; at high levels inhibits FSH and LH (negative feedback).
If no fertilisation: corpus luteum degenerates → progesterone falls → lining shed (menstruation) → FSH rises again.
Contraception
Hormonal methods:
- Combined oral contraceptive pill — oestrogen + progesterone; inhibits FSH production (no egg matures) and LH (no ovulation); maintains high progesterone (thick cervical mucus, thin lining).
- Progesterone-only pill — thickens cervical mucus; must be taken at same time each day.
- Injection/implant/IUS — long-acting progesterone.
Non-hormonal: condom (also STI protection), IUD (copper), diaphragm, abstinence.
Pros of hormonal: very reliable (>99% if used correctly); does not interrupt sex.
Cons: side effects (mood changes, blood clots); no STI protection; requires medical prescription.
IVF (in vitro fertilisation)
- Ovaries stimulated with FSH + LH injections to produce multiple eggs.
- Eggs collected and fertilised with partner's (or donor's) sperm in a lab.
- Embryos cultured for 2–5 days.
- One or two embryos implanted into the uterus.
Ethical considerations: multiple embryos created — unused ones destroyed; multiple pregnancy risks if >1 embryo implanted; expensive; low success rates (20–30% per cycle).
Common exam errors
- Confusing FSH and LH roles — FSH stimulates egg development; LH triggers ovulation.
- Saying progesterone rises in early cycle — it is produced only after ovulation by the corpus luteum.
- Forgetting that the combined pill works by inhibiting FSH (preventing egg maturation), not just preventing sperm entry.
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