Hormone systems - menstrual cycle

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

This page will answer many questions e.g.

What are hormones?

How does the menstrual cycle work?

How does In Vitro Fertilisation' (IVF) work?

How can hormones be used to control fertility?

What are FSH and LH hormones and what do they do?

  • You should appreciate that the nervous system and hormones enable us to respond to external changes.

  • The nervous system and hormones also help us to control conditions inside our bodies.

  • Know that hormones are used in some forms of contraception and in fertility treatments.

  • Know and understand that many process within the body are coordinated and controlled by chemical substances called hormones.

    • Know that hormones are secreted by glands and are usually transported to their target organs by the bloodstream.

    • Hormones, being directly released into the blood, are quite rapidly carried to all parts of the body BUT only affect the function of particular cells.

    • The activated cells are called 'target cells' and have a chemical receptor that responds to the hormone.

    • They are effectively act as 'chemical messages' to trigger particular biochemical reactions and their effect is relatively long-lasting compared to eg the nervous impulses and responses of reflex arc.

    • The pituitary gland produces the hormones FSH and LH which are important control chemicals in the female menstrual cycle.

      • The Menstrual Cycle

        • Stage 1 - the bleeding starts on day 1 as the uterus lining breaks down for 4 days.

        • Stage 2 - over 5 to 14 days the uterus builds up again to give a thickish layer of tissue to receive a fertilised egg.

        • Stage 3 - about day 14 an egg is released from the ovary

        • Stage 4 - the thickened uterus tissue lasts for around 14 days and if no fertilised egg is received the uterus lining breaks up and the cycle repeats itself.

          • You now need to know, as described below, the function of various hormones that control the different stages of the menstrual cycle.

    • The ovaries produces the hormone oestrogen, which is also involved in the menstrual cycle.

  • c) Know and understand that hormones regulate the functions of many organs and cells.

    • eg know the monthly release of an egg from a woman’s ovaries and the changes in the thickness of the lining of her womb are controlled by hormones secreted by the pituitary gland and by the ovaries.

  • d) Know and understand that several hormones are involved in the menstrual cycle of a woman and hormones are involved in promoting the release of an egg:

    • The follicle stimulating hormone (FSH) is secreted by the pituitary gland and causes eggs to mature in the ovaries and it also stimulates the ovaries to produce hormones including oestrogen,

    • The luteinising hormone (LH, also from the pituitary gland) stimulates the release of eggs from the ovary.

    • The hormone oestrogen is secreted by the ovaries, causes the formation of LH in the pituitary gland and inhibits the further production of FSH.

  • e) Know and understand about the uses of hormones in controlling fertility including:

    • giving oral contraceptives that contain hormones to inhibit FSH production so that no eggs mature, noting that ...

      • Oral contraceptives eg 'the pill' may contain oestrogen and progesterone to inhibit egg maturation ie prevent egg release, high and sustained levels of oestrogen inhibit FSH production and egg development is stopped permanently as long as the contraceptive is taken.

      • However, the first birth-control pills contained too large amounts of oestrogen which resulted in some women suffering significant side effects eg headaches, nausea, irregular bleeding, fluid retention and blood clots.

      • Birth-control pills now contain a much lower dose of oestrogen, or only contain another hormone called progesterone.

      • Progesterone-only pills lead to fewer side effects but is not quite as effective as oestrogen pills.

    • giving FSH and LH in a ‘fertility drug’ to a woman whose own level of FSH is too low to stimulate eggs to mature, for example in 'In Vitro Fertilisation' (IVF) treatment noting that ..

      • ... IVF involves giving a mother FSH and LH to stimulate the maturation of several eggs where they would not have been released.

        • IVF has enabled many mothers to get pregnant and give birth to healthy children, but it doesn't always work for an infertile couple.

        • Also, its possible for several eggs to be simultaneously matured resulting in multiple pregnancies eg twins, triplets and more!

        • Women can suffer side-effects from the hormone treatment which can be physically and emotionally demanding, but most infertile couples would regard IVF treatment is worth the risk.

      • In IVF treatment, after the potential mother has been treated with FSH and LH, eggs are collected from her ovaries and fertilised in the laboratory by sperm from the father.

      • The fertilised eggs grow and 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) to attempt a pregnancy.

    • Note that knowledge of the role of progesterone in the natural menstrual cycle, including details of negative feedback, is NOT required here.

  • You are expected to use appropriate skills, knowledge and understanding to:

    • evaluate the benefits of, and the problems that may arise from, the use of hormones to control fertility, including In Vitro Fertilisation (IVF) - you will be given information data to work on,

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