Human endocrine system

Students should be able to describe the principles of hormonal coordination and control by the human endocrine system.

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.

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.

Students should be able to identify the position of the following on a diagram of the human body:
- pituitary gland
- pancreas
- thyroid
- adrenal gland
- ovary
- testes.

Control of blood glucose concentration

Blood glucose concentration is monitored and controlled by the pancreas.

If the blood glucose concentration is too high, the pancreas produces the hormone insulin that causes glucose to move from the blood into the cells. In liver and muscle cells excess glucose is converted to glycogen for storage.

Students should be able to explain how insulin controls blood glucose (sugar) levels in the body.

Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin. It is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.

In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas. A carbohydrate controlled diet and an exercise regime are common treatments. Obesity is a risk factor for Type 2 diabetes.

Students should be able to compare Type 1 and Type 2 diabetes and explain how they can be treated.

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Students should be able to extract information and interpret data from graphs that show the effect of insulin in blood glucose levels in both people with diabetes and people without diabetes.

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If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.

Students should be able to explain how glucagon interacts with insulin in a negative feedback cycle to control blood glucose (sugar) levels in the body.

Maintaining water and nitrogen balance in the body

Students should be able to explain the effect on cells of osmotic changes in body fluids.

Water leaves the body via the lungs during exhalation.

Water, ions and urea are lost from the skin in sweat.

There is no control over water, ion or urea loss by the lungs or skin.

Excess water, ions and urea are removed via the kidneys in the urine.

If body cells lose or gain too much water by osmosis they do not function efficiently.

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The digestion of proteins from the diet results in excess amino acids which need to be excreted safely. In the liver these amino acids are deaminated to form ammonia. Ammonia is toxic and so it is immediately converted to urea for safe excretion.

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Students should be able to describe the function of kidneys in maintaining the water balance of the body.

The kidneys produce urine by filtration of the blood and selective reabsorption of useful substances such as glucose, some ions and water.

Knowledge of other parts of the urinary system, the structure of the kidney and the structure of a nephron is not required.

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Students should be able to translate tables and bar charts of glucose, ions and urea before and after filtration.

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Students should be able to describe the effect of ADH on the permeability of the kidney tubules.

The water level in the body is controlled by the hormone ADH which acts on the kidney tubules. ADH is released by the pituitary gland when the blood is too concentrated and it causes more water to be reabsorbed back into the blood from the kidney tubules. This is controlled by negative feedback.

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People who suffer from kidney failure may be treated by organ transplant or by using kidney dialysis. Students should know the basic principles of dialysis.

Hormones in human reproduction

Students should be able to describe the roles of hormones in human reproduction, including the menstrual cycle.

During puberty reproductive hormones cause secondary sex characteristics to develop.

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.

Testosterone is the main male reproductive hormone produced by the testes and it stimulates sperm production.

Several hormones are involved in the menstrual cycle of a woman.
- Follicle stimulating hormone (FSH) causes maturation of an egg in the ovary.
- Luteinising hormone (LH) stimulates the release of the egg.
- Oestrogen and progesterone are involved in maintaining the uterus lining.

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Students should be able to explain the interactions of FSH, oestrogen, LH and progesterone, in the control of the menstrual cycle.

Students should be able to extract and interpret data from graphs showing hormone levels during the menstrual cycle.

Contraception

Students should be able to evaluate the different hormonal and non-hormonal methods of contraception.

Fertility can be controlled by a variety of hormonal and non-hormonal methods of contraception.

These include:
- oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature
- injection, implant or skin patch of slow release progesterone to inhibit the maturation and release of eggs for a number of months or years
- barrier methods such as condoms and diaphragms which prevent the sperm reaching an egg
- intrauterine devices which prevent the implantation of an embryo or release a hormone
- spermicidal agents which kill or disable sperm
- abstaining from intercourse when an egg may be in the oviduct
- surgical methods of male and female sterilisation.

The use of hormones to treat infertility

Students should be able to explain the use of hormones in modern reproductive technologies to treat infertility.

This includes giving FSH and LH in a ‘fertility drug’ to a woman. She may then become pregnant in the normal way.

In Vitro Fertilisation (IVF) treatment.
- IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs.
- The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
- The fertilised eggs develop into embryos.
- At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb).

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Although fertility treatment gives a woman the chance to have a baby of her own:
- it is very emotionally and physically stressful
- the success rates are not high
- it can lead to multiple births which are a risk to both the babies and the mother.

Negative feedback

Students should be able to explain the roles of thyroxine and adrenaline in the body.

Adrenaline is produced by the adrenal glands in times of fear or stress. It increases the heart rate and boosts the delivery of oxygen and glucose to the brain and muscles, preparing the body for ‘flight or fight’.

Thyroxine from the thyroid gland stimulates the basal metabolic rate. It plays an important role in growth and development.

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Thyroxine levels are controlled by negative feedback.