Hormonal Coordination: The Endocrine System
The Endocrine System
The endocrine system is made up of glands that produce hormones. Hormones are chemical messengers carried in the blood to target organs, where they trigger a response.
Compared to the nervous system, hormonal responses are slower but longer-lasting and affect wider areas of the body.
Major Endocrine Glands
| Gland | Location | Hormone(s) produced | Function |
|---|---|---|---|
| Pituitary gland | Base of the brain | Many, including FSH, LH, ADH, growth hormone | "Master gland" — controls other glands; produces hormones that regulate many body functions |
| Thyroid | Neck | Thyroxine | Controls metabolic rate (how fast chemical reactions occur); regulates growth and development |
| Adrenal glands | On top of kidneys | Adrenaline | "Fight or flight" response — increases heart rate, breathing rate, blood flow to muscles |
| Pancreas | Behind the stomach | Insulin and glucagon | Controls blood glucose levels |
| Ovaries (females) | Pelvis | Oestrogen and progesterone | Control the menstrual cycle, development of secondary sexual characteristics, pregnancy |
| Testes (males) | Scrotum | Testosterone | Sperm production, development of male secondary sexual characteristics |
The pituitary gland is called the master gland because it produces hormones that act on other glands, stimulating them to release their own hormones.
Thyroxine and Negative Feedback
Thyroxine controls metabolic rate. It is regulated by negative feedback:
1. The pituitary gland releases TSH (thyroid-stimulating hormone)
2. TSH stimulates the thyroid to produce thyroxine
3. As thyroxine levels rise, the pituitary detects this and reduces TSH production
4. Less TSH → less thyroxine produced
5. When thyroxine levels fall, the pituitary increases TSH again
This is negative feedback — a change in a condition triggers a response that reverses the change, bringing it back to normal.
Adrenaline — Fight or Flight
Adrenaline is produced by the adrenal glands in response to danger, excitement, or stress. Its effects prepare the body for action:
- Increases heart rate — more blood pumped to muscles
- Increases breathing rate — more oxygen taken in
- Diverts blood flow to muscles and brain (away from digestive system)
- Releases stored glucose from glycogen in the liver — more fuel for muscles
- Dilates pupils — better vision
The Menstrual Cycle
The menstrual cycle is approximately 28 days and is controlled by four hormones interacting:
| Hormone | Produced by | Role |
|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Causes an egg to mature in one of the ovaries; stimulates ovaries to produce oestrogen |
| Oestrogen | Ovaries | Causes the uterus lining to thicken; stimulates release of LH; inhibits further FSH production |
| LH (luteinising hormone) | Pituitary gland | Triggers ovulation (release of the mature egg) on approximately day 14 |
| Progesterone | Ovaries (corpus luteum) | Maintains the thick uterus lining; inhibits both FSH and LH |
Timeline of the cycle:
- Day 1–5: Menstruation — uterus lining breaks down and is shed
- Day 5–14: Uterus lining rebuilds; egg matures (FSH + oestrogen rising)
- Day 14: Ovulation — egg released (triggered by LH surge)
- Day 14–28: Progesterone maintains lining; if the egg is not fertilised, progesterone drops → lining breaks down → cycle restarts
Hormonal Contraception
Hormonal methods use synthetic hormones to prevent pregnancy:
- Combined pill — contains oestrogen + progesterone; prevents ovulation by inhibiting FSH so no egg matures; over 99% effective
- Progesterone-only pill (mini pill) — thickens cervical mucus to prevent sperm reaching the egg
- Contraceptive injection/implant/patch — slow-release progesterone; lasts months to years
- IUS (intrauterine system) — releases progesterone inside the uterus
Non-hormonal methods:
- Condoms (male/female) — barrier method; also protects against STIs
- Diaphragm — placed over the cervix
- IUD (copper coil) — prevents implantation
- Surgical — vasectomy (male) or tubal ligation (female); permanent
- Abstinence or natural family planning (monitoring cycle)
Fertility Treatments
IVF (In Vitro Fertilisation):
1. The mother is given FSH and LH to stimulate multiple eggs to mature
2. Eggs are collected from the ovaries
3. Eggs are fertilised with sperm in a laboratory dish
4. The fertilised eggs develop into embryos
5. One or two embryos are transferred to the mother's uterus
6. If successful, an embryo implants and pregnancy begins
Issues with IVF:
- Emotionally and physically stressful
- Expensive — not always funded by the NHS
- Low success rate (~25–30% per cycle)
- Multiple embryos may implant → multiple births (health risks)
- Spare embryos may be destroyed — ethical concerns about the status of the embryo
Exam Tips
- Know the roles of all four menstrual cycle hormones (FSH, LH, oestrogen, progesterone) and which gland produces each
- Negative feedback is a common exam topic — use the thyroxine example
- For IVF, mention that FSH and LH are given to stimulate multiple egg maturation