Hormonal Coordination in Humans¶
Part of 4.5 Homeostasis and Response.
Hormones are chemical messengers carried in the blood. Compared with nervous impulses they usually act more slowly, but they can produce longer-lasting and body-wide effects.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
4.5.3-lo-1 |
4.5.3.1 Students should be able to describe the principles of hormonal coordination and control by the human endocrine system. 4.5.3.1 The endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream. The blood carries the hormone to a target organ where it produces an effect. Compared to the nervous system the effects are slower but act for longer. 4.5.3.1 The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions. These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects. 4.5.3.1 Students should be able to identify the position of the following on a diagram of the human body: 4.5.3.1 • pituitary gland 4.5.3.1 • pancreas 4.5.3.1 • thyroid 4.5.3.1 • adrenal gland 4.5.3.1 • ovary 4.5.3.1 • testes. |
The Endocrine System |
4.5.3-lo-2 |
4.5.3.7 Students should be able to explain the roles of thyroxine and adrenaline in the body. 4.5.3.7 Adrenaline is produced by the adrenal glands in times of fear or stress. 4.5.3.7 increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’. 4.5.3.7 Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development. 4.5.3.7 Thyroxine levels are controlled by negative feedback. |
Adrenaline: The 'Fight or Flight' Hormone, Thyroxine and Metabolic Rate |
4.5.3-lo-3 |
4.5.3.4 Students should be able to describe the roles of hormones in human reproduction, including the menstrual cycle. 4.5.3.4 During puberty reproductive hormones cause secondary sex characteristics to develop. 4.5.3.4 Oestrogen is the main female reproductive hormone produced in the ovary. At puberty eggs begin to mature and one is released approximately every 28 days. This is called ovulation. 4.5.3.4 Testosterone is the main male reproductive hormone produced by the testes and it stimulates sperm production. 4.5.3.4 Several hormones are involved in the menstrual cycle of a woman. 4.5.3.4 • Follicle stimulating hormone (FSH) causes maturation of an egg in the ovary. 4.5.3.4 • Luteinising hormone (LH) stimulates the release of the egg. 4.5.3.4 • Oestrogen and progesterone are involved in maintaining the uterus lining. 4.5.3.4 (HT only) Students should be able to explain the interactions of FSH, oestrogen, LH and progesterone, in the control of the menstrual cycle. 4.5.3.4 (HT only) Students should be able to extract and interpret data from graphs showing hormone levels during the menstrual cycle. |
Puberty and Reproductive Hormones, The Menstrual Cycle |
4.5.3-lo-4 |
4.5.3.5 Students should be able to evaluate the different hormonal and non- hormonal methods of contraception. 4.5.3.5 Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception. 4.5.3.5 These include: 4.5.3.5 • oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature 4.5.3.5 • injection, implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months or years 4.5.3.5 • barrier methods such as condoms and diaphragms which prevent the sperm reaching an egg 4.5.3.5 • intrauterine devices which prevent the implantation of an embryo or release a hormone 4.5.3.5 • spermicidal agents which kill or disable sperm 4.5.3.5 • abstaining from intercourse when an egg may be in the oviduct 4.5.3.5 • surgical methods of male and female sterilisation. |
Contraception |
4.5.3-lo-5 |
4.5.3.6 Students should be able to explain the use of hormones in modern reproductive technologies to treat infertility. 4.5.3.6 This includes giving FSH and LH in a ‘fertility drug’ to a woman. She may then become pregnant in the normal way. 4.5.3.6 In Vitro Fertilisation (IVF) treatment. 4.5.3.6 • IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs. 4.5.3.6 • The eggs are collected from the mother and fertilised by sperm from the father in the laboratory. 4.5.3.6 • The fertilised eggs develop into embryos. 4.5.3.6 • At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb). 4.5.3.6 Although fertility treatment gives a woman the chance to have a baby her own: 4.5.3.6 • it is very emotionally and physically stressful 4.5.3.6 • the success rates are not high 4.5.3.6 • it can lead to multiple births which are a risk to both the babies and the mother. |
Treating Infertility |
The Endocrine System¶
Endocrine glands release hormones directly into the bloodstream rather than via ducts. Hormones travel throughout the body but only affect cells that have the correct receptors (target cells).
Key endocrine glands and what they produce:
| Gland | Hormones |
|---|---|
| Pituitary gland (brain) | FSH, LH, ADH, growth hormone |
| Thyroid gland (throat) | Thyroxine |
| Adrenal glands (above kidneys) | Adrenaline |
| Pancreas (abdomen) | Insulin, glucagon |
| Ovaries | Oestrogen, progesterone |
| Testes | Testosterone |
Adrenaline: The 'Fight or Flight' Hormone¶
Adrenaline is produced by the adrenal glands in response to stress or danger. It prepares the body for rapid action:
- Increases heart rate and blood pressure — delivers more oxygen to muscles.
- Stimulates breakdown of glycogen to glucose in the liver — provides quick energy.
- Dilates the pupils — improves awareness.
- Widens the bronchioles — allows faster gas exchange.
- Causes vasoconstriction in the gut — redirects blood to muscles and brain.
This suite of effects is called the fight-or-flight response.
Thyroxine and Metabolic Rate¶
Thyroxine is produced by the thyroid gland in the throat. It controls the basal metabolic rate (the body's resting rate of metabolism) and also plays a role in growth and development.
Thyroxine levels are kept stable by a negative feedback loop involving the hypothalamus and pituitary gland:
- When thyroxine is low: the hypothalamus releases TRH → pituitary releases TSH → TSH stimulates the thyroid to produce more thyroxine → levels rise.
- When thyroxine is normal/high: high thyroxine inhibits TRH and TSH release → thyroid produces less thyroxine → levels fall.
Hypothyroidism (too little thyroxine): weight gain, slow heart rate, fatigue, reduced fertility. Hyperthyroidism (too much thyroxine): weight loss, palpitations, irritability, irregular menstrual cycle. Both conditions can cause a goitre (swelling in the neck).
Puberty and Reproductive Hormones¶
At puberty, the body begins producing sex hormones that drive the development of secondary sex characteristics and the ability to produce sex cells.
- Testosterone (produced by the testes):
- Facial, body and pubic hair growth.
- Muscle development.
- Voice deepening.
-
Begins sperm production.
-
Oestrogen (produced by the ovaries):
- Breast development.
- Widening of hips.
- Underarm and pubic hair growth.
- Stimulates the menstrual cycle and egg maturation.
The Menstrual Cycle¶
The menstrual cycle is approximately 28 days long. Its purpose is to prepare an egg for potential fertilisation. Four hormones control it, two from the pituitary gland and two from the ovaries.
| Hormone | Source | Timing | Role |
|---|---|---|---|
| FSH (follicle-stimulating hormone) | Pituitary gland | Days 1–13 | Stimulates growth of the follicle and egg maturation |
| Oestrogen | Ovaries | Days 4–15 | Rebuilds the uterus lining; at high levels, triggers LH surge and inhibits FSH |
| LH (luteinising hormone) | Pituitary gland | Day 14 (surge) | Triggers ovulation (release of the egg from the follicle) |
| Progesterone | Follicle (corpus luteum) | Days 15–27 | Maintains uterus lining; inhibits FSH and LH |
Cycle summary: 1. Days 1–5: Menstruation — old uterus lining breaks down. 2. Days 2–13: FSH stimulates follicle growth and egg maturation; oestrogen rebuilds the uterus lining. 3. Day ~14: Oestrogen surge triggers an LH surge → ovulation (egg released). 4. Days 15–27: Progesterone from the ruptured follicle (corpus luteum) maintains the uterus lining. 5. Day 28: If no fertilisation, progesterone drops, FSH inhibition ends, uterus lining breaks down — cycle begins again.
During pregnancy: oestrogen and progesterone remain high (to maintain the uterus lining and prepare for milk production). FSH and LH are suppressed.
Hormonal Interactions (Feedback)¶
- High oestrogen → inhibits FSH (negative feedback) and stimulates a surge of LH (positive feedback at this point).
- High progesterone → inhibits both FSH and LH.
- Falling oestrogen and progesterone → FSH inhibition ends → new follicle can be stimulated → cycle restarts.
Contraception¶
Contraception aims to prevent fertilisation. Methods include:
Hormonal contraception: - Combined oral contraceptive pill — contains oestrogen and progesterone at levels that suppress FSH and LH, preventing egg maturation and ovulation. - Progesterone-only methods — injections, implants, patches, or hormonal IUD. Progesterone thickens cervical mucus (blocking sperm) and prevents egg maturation. Success rate 92–99% if used correctly. - Side effects can include weight gain, acne, mood changes, and changes in blood pressure.
Non-hormonal methods include condoms (also protect against sexually transmitted infections), diaphragms, and IUDs.
Treating Infertility¶
Infertility can result from insufficient FSH production, blocked oviducts, or problems with sperm. Treatments include:
Fertility drugs: FSH and LH are given to stimulate follicle development and egg maturation, increasing the chance of ovulation and natural fertilisation.
In vitro fertilisation (IVF): 1. The woman is given FSH and LH drugs to stimulate multiple eggs to mature. 2. Eggs are collected from the ovaries; sperm is obtained from the partner. 3. Eggs and sperm are mixed in the laboratory to allow fertilisation. 4. Fertilised eggs develop into embryos. 5. One or two healthy embryos are selected and placed into the mother's uterus. 6. If successful, the embryo implants and a normal pregnancy follows.
Artificial insemination (AI): sperm is inserted directly into the uterus, fallopian tubes or cervix. Useful when intercourse is not possible or when using donor sperm.
Ethical considerations: IVF can result in multiple pregnancies (health risks); unused embryos raise ethical questions; treatments can be emotionally and physically demanding.
Common Confusions¶
- FSH vs LH: FSH stimulates the follicle and egg maturation earlier in the cycle; LH triggers ovulation. Students often swap these.
- Oestrogen's dual role: oestrogen both inhibits FSH (negative feedback) and stimulates LH (positive feedback to cause ovulation). This seems contradictory but it does both.
- The pill works by mimicking pregnancy: keeping oestrogen and progesterone high prevents FSH from stimulating a new follicle — similar to how pregnancy suppresses the cycle.
- Thyroxine feedback: low thyroxine → TRH → TSH → more thyroxine. Students often confuse the direction of these signals.
Key Terms¶
- Hormone: a chemical messenger released by a gland and carried in the blood.
- Endocrine gland: a gland that releases hormones directly into the bloodstream.
- Insulin: a hormone that lowers blood glucose concentration.
- Glucagon: a hormone that raises blood glucose concentration.
- ADH: a hormone that increases water reabsorption in the kidneys.
- Negative feedback: control in which a change away from the ideal level triggers responses that reverse the change.
- Adrenaline: a hormone released by the adrenal glands in response to stress; prepares the body for fight or flight.
- Thyroxine: a hormone from the thyroid gland that controls basal metabolic rate.
- Testosterone: the main male sex hormone, produced by the testes; responsible for male secondary sex characteristics and sperm production.
- Oestrogen: the main female sex hormone, produced by the ovaries; controls female secondary sex characteristics and the menstrual cycle.
- Progesterone: a female reproductive hormone (produced by the corpus luteum after ovulation) that maintains the uterus lining.
- FSH (follicle-stimulating hormone): a pituitary hormone that stimulates follicle development and egg maturation.
- LH (luteinising hormone): a pituitary hormone that triggers ovulation.
- Ovulation: the release of a mature egg from the ovary; occurs around day 14 of the menstrual cycle.
- IVF (in vitro fertilisation): a fertility treatment in which eggs and sperm are combined in a laboratory and resulting embryos are implanted into the uterus.
- Basal metabolic rate: the body's minimum rate of energy use at rest; controlled partly by thyroxine.
- Fight-or-flight response: the set of physiological changes triggered by adrenaline in response to perceived danger.