What you'll learn

• What hormones are and how the endocrine system coordinates the body.
• How adrenaline and thyroxine affect the body, including thyroxine negative feedback at Higher tier.
• How hormones control human reproduction and the menstrual cycle.
• How contraception and fertility treatments use knowledge of hormones.

Big picture: why the body needs coordination

Your body has to coordinate lots of processes at the same time: growth, metabolism, stress responses, reproduction, and keeping internal conditions steady.

There are two main coordination systems:

• the nervous system, which uses electrical impulses through neurones and is usually very fast
• the endocrine system, which uses hormones carried in the blood and is usually slower but longer-lasting

The endocrine system

The endocrine system is made of endocrine glands. A gland is an organ that secretes a useful substance. Endocrine glands secrete hormones directly into the bloodstream.

Important human endocrine glands include:

• the pituitary gland in the brain
• the thyroid gland in the neck
• the adrenal glands above the kidneys
• the pancreas
• the ovaries in females
• the testes in males

Hormones travel all around the body in the blood, but only some cells respond. These are target cells. A target cell has the correct receptor, which is a molecule with a shape that the hormone can bind to.

Hormonal vs nervous coordination

Adrenaline and thyroxine

At Higher tier, you need to be able to explain the roles of adrenaline and thyroxine, not just name them.

Adrenaline

Adrenaline is a hormone released by the adrenal glands. It prepares the body for a “fight or flight” response when you are frightened, excited or stressed.

Adrenaline can:

• increase heart rate, so more oxygen and glucose are delivered to muscles
• increase breathing rate, so more oxygen enters the blood
• increase blood glucose concentration, giving cells more fuel for respiration
• redirect blood flow towards muscles

Thyroxine

Thyroxine is a hormone released by the thyroid gland. It helps control the body’s metabolic rate, which means the rate of chemical reactions in cells. Thyroxine also affects growth and development.

Thyroxine levels are controlled by negative feedback.

For thyroxine:

• If thyroxine level is too low, the pituitary gland releases more TSH.
• TSH stands for thyroid-stimulating hormone. It stimulates the thyroid gland to release more thyroxine.
• If thyroxine level becomes too high, the pituitary releases less TSH.
• Less TSH means the thyroid releases less thyroxine.

Hormones in human reproduction

Human reproduction is controlled by several hormones. Some are involved in puberty, sperm production, egg maturation and the menstrual cycle.

Key reproductive hormones

FSH stands for follicle-stimulating hormone. LH stands for luteinising hormone.

The menstrual cycle

The menstrual cycle is a repeating cycle of changes in the ovaries and uterus. A typical cycle is about 28 days, but real cycles vary between people and from month to month.

The cycle starts on day 1, the first day of menstruation. Menstruation is when the uterus lining breaks down and leaves the body.

Ovulation is the release of an egg from an ovary. In a typical 28-day cycle, this happens around day 14.

The graph below shows the relative changes in the main menstrual cycle hormones. Focus on the pattern, especially the LH peak at ovulation and the rise in progesterone after ovulation.

The hormone interactions

At Higher tier, you need to explain how FSH, LH, oestrogen and progesterone interact.

1. FSH is released by the pituitary gland. It causes an egg to mature in a follicle in the ovary.
2. The developing follicle releases oestrogen.
3. Oestrogen causes the uterus lining to thicken, preparing it for possible implantation of an embryo.
4. Oestrogen also reduces FSH release, so usually only one egg matures.
5. When oestrogen becomes high enough, it stimulates a sharp rise in LH.
6. The LH surge triggers ovulation.
7. After ovulation, the ovary releases progesterone.
8. Progesterone maintains the thick uterus lining and inhibits FSH and LH.
9. If pregnancy does not happen, progesterone falls and the uterus lining breaks down, causing menstruation.

Contraception

Contraception means methods used to reduce the chance of pregnancy.

Contraception can be hormonal or non-hormonal.

Hormonal contraception

Hormonal contraception uses synthetic versions of reproductive hormones. These can prevent pregnancy by:

• stopping FSH release, so an egg does not mature
• stopping the LH surge, so ovulation does not happen
• thickening mucus at the cervix, making it harder for sperm to reach an egg
• making the uterus lining less suitable for implantation

Examples include:

• the contraceptive pill
• patches
• injections
• implants
• hormonal intrauterine systems, often called IUS

Long-acting methods such as implants and IUS are usually very effective because the user does not have to remember them every day.

Non-hormonal contraception

Non-hormonal methods do not use hormones. Examples include:

• condoms, which act as a barrier and also reduce the spread of sexually transmitted infections, or STIs
• diaphragms, which cover the cervix
• copper intrauterine devices, often called copper IUDs, which mainly prevent fertilisation
• sterilisation, a surgical method intended to be permanent
• natural family planning, which avoids sex around the most fertile days, but is less reliable because cycles vary

Fertility treatments

At Higher tier, you also need to explain how hormones are used in modern reproductive technologies to treat infertility. Infertility means difficulty becoming pregnant despite regular unprotected sex.

Fertility drugs

Fertility drugs can contain hormones such as FSH and LH. These stimulate eggs to mature and can trigger ovulation. This may help if infertility is caused by eggs not maturing or ovulation not happening regularly.

IVF

IVF stands for in vitro fertilisation. “In vitro” means outside the body, in laboratory conditions.

A simplified IVF process is:

1. The woman is given hormones such as FSH and LH to stimulate several eggs to mature.
2. Eggs are collected from the ovaries.
3. The eggs are fertilised with sperm in a laboratory.
4. Embryos are grown for a short time.
5. One or more embryos are transferred into the uterus.

IVF can help people have a baby, but it is not guaranteed to work. It can be physically and emotionally stressful. There may also be ethical concerns about unused embryos, and transferring more than one embryo can increase the chance of multiple births, which carry extra health risks.