Hormones and the endocrine system

What you'll learn

This revision guide covers the endocrine system and hormonal coordination as examined in AQA GCSE Biology. You'll learn how hormones act as chemical messengers, how different glands produce specific hormones, and how these systems regulate key body functions including blood glucose levels and reproduction. This topic appears in both Foundation and Higher tier papers and typically accounts for 6-8 marks per exam.

Key terms and definitions

Hormone — A chemical messenger secreted by endocrine glands that travels in the blood to target organs, producing specific effects.

Endocrine gland — A gland that secretes hormones directly into the bloodstream (ductless glands).

Target organ — An organ that contains cells with specific receptors for a particular hormone, allowing it to respond to that hormone.

Negative feedback — A control mechanism where a change in a system triggers a response that counteracts the original change, maintaining homeostasis.

Menstrual cycle — The monthly reproductive cycle in females, controlled by hormones, involving the thickening and shedding of the uterus lining.

Insulin — A hormone produced by the pancreas that reduces blood glucose concentration by causing glucose to move from blood into cells.

Glucagon — A hormone produced by the pancreas that increases blood glucose concentration by triggering the conversion of glycogen to glucose in the liver.

Oestrogen — A female sex hormone produced by the ovaries that stimulates the uterus lining to thicken and inhibits FSH production.

Core concepts

The endocrine system vs the nervous system

The human body uses two coordination systems to respond to changes:

The nervous system:

• Uses electrical impulses travelling along neurones
• Very fast response (milliseconds)
• Short-lasting effects
• Acts on specific, precise locations

The endocrine system:

• Uses chemical messengers (hormones) in the bloodstream
• Slower response (seconds to hours)
• Long-lasting effects
• Can affect multiple target organs throughout the body

Both systems work together to maintain homeostasis and coordinate body functions.

Major endocrine glands and their hormones

You must know the location and function of these glands for AQA exams:

Pituitary gland (in the brain):

• Called the "master gland" because it secretes several hormones that control other glands
• Produces FSH (follicle-stimulating hormone) and LH (luteinising hormone) for reproduction
• Produces growth hormone
• Produces ADH (antidiuretic hormone) for water balance

Thyroid gland (in the neck):

• Produces thyroxine
• Controls metabolic rate, heart rate and temperature

Pancreas (in the abdomen):

• Produces insulin and glucagon
• Controls blood glucose concentration

Adrenal glands (above the kidneys):

• Produce adrenaline
• Prepares the body for "fight or flight" responses

Ovaries (females only):

• Produce oestrogen and progesterone
• Control the menstrual cycle and development of female secondary sexual characteristics

Testes (males only):

• Produce testosterone
• Controls sperm production and development of male secondary sexual characteristics

Control of blood glucose concentration

Blood glucose regulation is a key example of homeostasis and negative feedback. The pancreas constantly monitors blood glucose levels.

When blood glucose is too high:

1. Pancreas detects high blood glucose
2. Pancreas secretes insulin
3. Insulin causes glucose to move from blood into cells (especially liver and muscle cells)
4. In liver and muscle cells, excess glucose is converted to glycogen for storage
5. Blood glucose concentration decreases back to normal

When blood glucose is too low:

1. Pancreas detects low blood glucose
2. Pancreas secretes glucagon
3. Glucagon causes glycogen in liver cells to be converted back to glucose
4. Glucose is released into the bloodstream
5. Blood glucose concentration increases back to normal

This demonstrates negative feedback — the response reverses the original change to maintain a stable internal environment.

Type 1 diabetes:

• The pancreas produces little or no insulin
• Blood glucose can rise to fatally high levels
• Usually develops in childhood/early adulthood
• Treatment: insulin injections throughout the day, especially before meals
• Patients must monitor blood glucose levels and limit simple carbohydrate intake

Type 2 diabetes:

• Body cells no longer respond properly to insulin (insulin resistance)
• Often linked to obesity and lack of exercise
• Usually develops in older adults, but increasingly seen in younger people
• Treatment: lifestyle changes (diet, exercise, weight loss); medication if needed
• Can often be prevented or managed through healthy lifestyle choices

The menstrual cycle

The menstrual cycle lasts approximately 28 days and is controlled by four hormones working in sequence:

Day 1-5 (menstruation):

• Uterus lining breaks down and is shed through the vagina
• This is the menstrual period

Hormones controlling the cycle:

FSH (Follicle-Stimulating Hormone):

• Produced by the pituitary gland
• Causes an egg to mature in one of the ovaries (inside a follicle)
• Stimulates the ovaries to produce oestrogen

Oestrogen:

• Produced by the ovaries
• Causes the uterus lining to thicken and become maintained with blood vessels ready for a fertilised egg
• Inhibits (stops) further FSH production
• Stimulates the pituitary to release LH

LH (Luteinising Hormone):

• Produced by the pituitary gland
• Stimulates ovulation (release of a mature egg) around day 14
• Stimulates the remains of the follicle to develop into a corpus luteum, which secretes progesterone

Progesterone:

• Produced by the corpus luteum in the ovaries (after ovulation)
• Maintains the thick uterus lining
• Inhibits FSH and LH production
• If no fertilisation occurs, progesterone levels fall and the cycle repeats

Contraception methods

Hormonal contraceptives prevent pregnancy by controlling the menstrual cycle:

Oral contraceptives (the pill):

• Contains oestrogen and progesterone
• Inhibits FSH production so no eggs mature
• Over 99% effective if taken correctly
• Advantages: very effective, can reduce period pain, reduces risk of some cancers
• Disadvantages: must remember daily, doesn't protect against STIs, small risk of blood clots

Contraceptive injection, implant, or skin patch:

• Contains progesterone only (or progesterone and oestrogen)
• Slowly releases hormones over weeks or months
• Prevents ovulation and thickens cervical mucus
• Advantages: long-lasting, don't need to remember daily
• Disadvantages: can cause irregular periods, doesn't protect against STIs

Non-hormonal contraceptives:

• Barrier methods (condoms, diaphragm): prevent sperm reaching egg; condoms protect against STIs
• IUD (intrauterine device): copper device prevents implantation
• Natural family planning: avoiding intercourse during fertile period
• Sterilisation: cutting or blocking fallopian tubes/sperm ducts (permanent)
• Abstinence: not having sexual intercourse

Fertility treatments

Fertility drugs help couples who have difficulty conceiving:

FSH and LH treatment:

• Given to women who don't produce enough FSH naturally
• Stimulates egg maturation and ovulation
• Advantages: enables pregnancy, uses natural hormones
• Disadvantages: can cause multiple pregnancies (twins, triplets), which carries higher risks for mother and babies

IVF (In Vitro Fertilisation):

1. Mother given FSH and LH to stimulate multiple eggs to mature
2. Eggs collected from ovaries
3. Eggs fertilised with father's sperm in a laboratory
4. Fertilised eggs develop into embryos
5. One or two embryos inserted into mother's uterus

Advantages: can help many infertile couples; unused embryos can be frozen for later attempts

Disadvantages: emotionally and physically stressful; success rate around 26% per cycle for women under 35; expensive; ethical concerns about unused embryos; multiple birth risks

Thyroxine and the negative feedback mechanism

Thyroxine regulates metabolic rate through negative feedback:

1. Pituitary gland releases TSH (thyroid-stimulating hormone)
2. TSH causes thyroid gland to release thyroxine
3. Thyroxine increases metabolic rate
4. When thyroxine levels are high enough, thyroxine inhibits TSH release from pituitary
5. Less TSH means less thyroxine is released
6. When thyroxine levels drop, inhibition decreases and TSH release increases again

This maintains thyroxine at the correct level.

Adrenaline and the fight or flight response

Adrenaline prepares the body for emergency action:

• Released by adrenal glands in response to fear, stress or excitement
• Increases heart rate and blood pressure
• Diverts blood flow to muscles and away from digestive system
• Increases blood glucose concentration by triggering glycogen breakdown
• Dilates pupils
• Increases breathing rate

These changes prepare the body for rapid physical activity.

Worked examples

Example 1: Blood glucose regulation (4 marks)

Question: Explain how the body responds when blood glucose concentration becomes too high after eating a meal. (4 marks)

Mark scheme answer:

• High blood glucose is detected by the pancreas (1 mark)
• The pancreas secretes/releases insulin (1 mark)
• Insulin causes glucose to move from blood into cells / into liver and muscle cells (1 mark)
• Excess glucose is converted into glycogen for storage (1 mark)

Examiner note: Each biological fact earns one mark. You must mention the specific hormone (insulin, not just "hormones") and the organs involved.

Example 2: Menstrual cycle hormones (6 marks)

Question: Describe the roles of FSH and LH in the menstrual cycle. (6 marks)

Mark scheme answer:

FSH:

• Produced/secreted by the pituitary gland (1 mark)
• Causes an egg to mature in the ovary / develops follicles (1 mark)
• Stimulates ovaries to produce oestrogen (1 mark)

LH:

• Produced/secreted by the pituitary gland (1 mark)
• Stimulates ovulation / release of the egg (1 mark)
• (At day 14 / middle of cycle) (additional detail — 1 mark)

Examiner note: "Describe" questions require you to state facts without detailed explanation. Include where hormones are produced and what they do.

Example 3: Diabetes comparison (4 marks)

Question: Compare Type 1 and Type 2 diabetes. (4 marks)

Mark scheme answer:

• Type 1: pancreas produces little/no insulin; Type 2: body cells don't respond to insulin / insulin resistance (1 mark)
• Type 1 usually develops in childhood/young people; Type 2 usually develops in adults (1 mark)
• Type 1 treated with insulin injections; Type 2 treated with lifestyle changes / diet and exercise (1 mark)
• Type 2 often linked to obesity/lack of exercise; Type 1 is not (1 mark)

Examiner note: "Compare" means identify similarities AND differences. Present answers in paired statements for clarity.

Common mistakes and how to avoid them

• Confusing glands and organs. Remember that endocrine glands produce hormones (pancreas, pituitary, ovaries), while target organs respond to hormones. The liver is a target organ for insulin, not the gland that makes it.

• Saying hormones travel through the nervous system. Hormones travel in the bloodstream, not along nerves. Electrical impulses travel along neurones.

• Mixing up insulin and glucagon effects. Insulin lowers blood glucose (remember: insulin → in → glucose goes into cells). Glucagon raises blood glucose (converts glycogen → glucose).

• Stating FSH or LH "makes eggs". Be precise: FSH causes eggs to mature, not to be made. Females are born with all their eggs already present.

• Writing that oestrogen is released at ovulation. LH is released at ovulation (day 14). Oestrogen builds up before ovulation and stimulates LH release.

• Forgetting that the contraceptive pill contains hormones. Don't just say "it prevents pregnancy" — explain that oestrogen/progesterone inhibit FSH so eggs don't mature.

Exam technique for "Hormones and the endocrine system"

• Command word precision. "Explain" requires reasons (use "because", "so", "therefore"). "Describe" needs facts without reasons. "Compare" needs both similarities and differences, not just one type.

• Name specific hormones and glands. Avoid vague terms like "hormones are released" — state which hormone and from which gland. Examiners cannot award marks for imprecise biology.

• Use correct sequence in cycle questions. For menstrual cycle or negative feedback, number your points or use clear sequence words (first, then, next, this causes) to show the order of events. Marks are often awarded for correct sequence.

• Link structure to function. When explaining why hormones are slower than nerves, mention that blood flow is slower than electrical impulses. Connect the biological mechanism to the outcome.

Quick revision summary

The endocrine system uses hormones as chemical messengers released by glands into the bloodstream to reach target organs. Key examples include insulin and glucagon from the pancreas controlling blood glucose through negative feedback, and FSH, LH, oestrogen and progesterone controlling the menstrual cycle. Type 1 diabetes requires insulin injections while Type 2 is often managed through lifestyle. Hormonal contraceptives work by inhibiting FSH. The endocrine system produces slower, longer-lasting effects than the nervous system.