Homeostasis: thermoregulation and reproductive hormones

Homeostasis maintains stable internal conditions despite changes outside.

Thermoregulation

Core body temperature is held near 37 degrees C by the thermoregulatory centre in the brain (hypothalamus). Receptors in the skin send information about external temperature.

Too hot	Too cold
Vasodilation — blood vessels near skin widen, more heat lost by radiation	Vasoconstriction — vessels narrow, less heat lost
Sweating — evaporation cools the skin	Shivering — muscles contract rapidly, releasing heat from respiration
Hairs lie flat	Hairs stand up (erector muscles contract) trapping insulating air

The menstrual cycle (~28 days)

Four hormones, two glands:

Hormone	Made in	Effect
FSH	Pituitary	Causes egg to mature in the ovary; stimulates ovaries to make oestrogen
Oestrogen	Ovary	Builds up the uterus lining; inhibits FSH; triggers LH
LH	Pituitary	Triggers ovulation around day 14
Progesterone	Ovary (corpus luteum)	Maintains the lining; inhibits FSH and LH

If no fertilisation, progesterone falls and the lining is shed (menstruation).

Contraception

Method	How it works
Combined pill (oestrogen + progesterone)	Inhibits FSH so no eggs mature
Condom / diaphragm	Barrier preventing sperm reaching egg
IUD (coil)	Prevents implantation
Sterilisation	Cutting/tying tubes
Natural / abstinence	Avoiding sex during fertile window

IVF (in vitro fertilisation)

1. Mother is given FSH and LH to mature several eggs.
2. Eggs are collected and fertilised by sperm in the lab.
3. Embryos are grown to the 8-cell stage and one or two are inserted into the uterus.

Pros: can help couples with infertility. Cons: expensive, low success rate, can lead to multiple births and emotional stress.

WJEC exam tip

A common Higher question gives a hormone-level graph and asks you to identify the day. Use these anchors: FSH peaks early (day 1-5), oestrogen peaks just before ovulation (day 12-13), LH spikes around day 14, progesterone peaks around day 21.