Hormonal Coordination in Humans¶

Part of 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.

What You Need to Learn¶

Explore how blood-borne signals coordinate organs, prepare the body for action and regulate metabolism, blood glucose and water balance. Then connect reproductive hormones to puberty, the menstrual cycle, contraception and fertility treatment. Compare the benefits and limitations of these treatments; Higher Tier work includes fertility treatment, adrenaline and thyroxine, interpreting hormone graphs, and explaining feedback between reproductive hormones.

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).

The pituitary gland at the base of the brain releases several hormones that stimulate other endocrine glands, linking body conditions to coordinated responses. The ovaries lie in the lower abdomen and the testes lie in the scrotum.

Key endocrine glands and what they release:

Adrenaline: The 'Fight or Flight' Hormone — Higher Tier¶

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 — Higher Tier¶

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).

Control of Blood Glucose¶

The pancreas monitors blood glucose. After a meal, a rise triggers release of insulin. Insulin promotes glucose uptake from the blood into cells, and excess glucose is stored as glycogen in liver and muscle cells. Blood glucose therefore falls towards its usual range.

For Higher Tier, a fall in blood glucose triggers glucagon release from the pancreas. Glucagon causes the liver to convert glycogen to glucose and release it into the blood. Insulin and glucagon oppose changes in either direction: as the normal range is restored, the corrective hormone response decreases. This is negative feedback.

In Type 1 diabetes, the pancreas does not release enough insulin, so blood glucose can remain too high. Insulin injections replace the missing hormone. In Type 2 diabetes, body cells respond less effectively to insulin. Managing carbohydrate intake and taking exercise can help control glucose levels; obesity increases risk but does not mean everyone with obesity develops diabetes.

When interpreting blood-glucose graphs, compare the starting concentration, the size of the rise after food and the time taken to return towards the starting range. Use the axes and units to quantify differences. A prolonged high concentration can indicate ineffective control, but one graph alone does not establish the cause.

Water Balance and the Kidneys¶

Water leaves the body in exhaled air, while sweat carries water, ions and some urea. These losses are not adjusted to maintain water balance. The kidneys regulate what remains by removing excess water, ions and urea in urine. If body fluids become too concentrated, cells lose water by osmosis; if they become too dilute, cells gain water. Either excessive change disrupts cell function.

The kidneys first filter small substances from blood, including glucose, water, ions and urea. They then selectively reabsorb useful substances: normally all glucose, and the required amounts of water and ions, return to the blood. Urea and substances not reabsorbed leave in urine. To interpret a table or bar chart, compare the amount filtered with the amount reabsorbed: the difference is the amount excreted, using consistent units.

For Higher Tier, excess amino acids cannot be stored as a reserve. The liver removes their nitrogen-containing groups by deamination, producing toxic ammonia. This is promptly converted to urea, which travels to the kidneys for excretion.

Also for Higher Tier, ADH released from the pituitary gland increases the permeability of kidney tubules to water. When blood becomes too concentrated, more ADH is released, more water returns to the blood and a smaller volume of concentrated urine forms. When blood is more dilute, less ADH is released, less water is reabsorbed and a larger volume of dilute urine forms. The correction reduces the stimulus for further ADH release: another negative-feedback loop.

Kidney Failure: Dialysis or Transplant¶

If kidneys fail, urea and inappropriate amounts of water or ions can accumulate. In dialysis, blood flows alongside dialysis fluid across a partially permeable membrane. The fluid contains suitable concentrations of useful glucose and ions but no urea. Urea diffuses out of the blood, while the concentration gradient removes excess ions; useful substances are retained. Water removal is controlled during treatment. Blood cells and large proteins remain in the blood because they cannot cross the membrane.

Dialysis can keep a person alive without a donor, but requires repeated, time-consuming treatment and careful management of diet and fluids. A kidney transplant can restore continuous kidney function and reduce these restrictions, but requires a suitable donor and surgery. Rejection is a risk, and medicines that suppress immune responses increase susceptibility to infection. Compare these practical, medical and social consequences when evaluating the options.

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¶

A 28-day menstrual cycle is a useful model; actual cycle lengths and the day of ovulation vary. Its purpose is to prepare an egg for potential fertilisation. Four hormones control it, two from the pituitary gland and two from the ovaries.

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) — Higher Tier¶

• 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 reduces the chance of pregnancy. Different methods act at different stages, so evaluate their effectiveness, duration, practical use and side effects. Methods include:

Hormonal contraception:

• Combined oral contraceptive pill — contains oestrogen and progesterone at levels that suppress FSH and LH, preventing egg maturation and ovulation.
• Progestogen methods — injections, implants and hormonal intrauterine devices release a synthetic hormone with progesterone-like effects. Depending on the method, these inhibit ovulation and/or thicken cervical mucus, making sperm passage more difficult. Hormonal skin patches can also prevent egg maturation and release. Longer-acting methods reduce the need to remember a daily dose.
• Side effects can include weight gain, acne, mood changes, and changes in blood pressure.

Non-hormonal methods:

• Condoms and diaphragms act as barriers preventing sperm reaching an egg. Condoms also reduce transmission of sexually transmitted infections.
• Copper intrauterine devices affect sperm and reduce the chance of fertilisation. Intrauterine methods may also make the uterus unsuitable for implantation.
• Spermicides kill or disable sperm, commonly alongside a barrier.
• Avoiding intercourse around ovulation reduces the chance of sperm meeting an egg, but variable cycle timing makes this difficult to predict.
• Sterilisation blocks sperm ducts or oviducts, preventing sperm and eggs meeting. It is intended to be permanent, which matters when comparing it with reversible methods.

Treating Infertility — Higher Tier¶

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.

Benefits and limitations: treatment can enable a pregnancy, but success is uncertain and repeated attempts can be emotionally and physically demanding. Multiple pregnancies increase risks to the parent and babies. Decisions about unused embryos also raise ethical questions.

For Higher Tier hormone graphs, identify peaks and changes over time rather than memorising dates alone: an LH peak precedes ovulation, while progesterone rises afterwards. Compare the timing of changes in different curves and connect them to follicle growth or maintenance of the uterus lining.

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 triggers responses that oppose it and restore the normal range.
• 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 transferred 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.

Connected Pages¶

• The Human Nervous System
• Plant Hormones
• Biology Key Terms Glossary