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examBoard: Pearson Edexcel
examType: IGCSE
lessonTitle: Sex Hormones
Sex hormones are chemical messengers that play a crucial role in human reproduction and sexual development. They control everything from puberty to pregnancy and are essential for the continuation of our species. In this guide, we'll explore how these powerful chemicals coordinate reproductive functions in the human body.
Key Definitions:
The main female sex hormones are oestrogen and progesterone. These are produced primarily in the ovaries. Oestrogen is responsible for the development of female secondary sexual characteristics during puberty, such as breast development and widening of the hips. Both hormones play vital roles in regulating the menstrual cycle and preparing the body for possible pregnancy.
Testosterone is the principal male sex hormone, produced mainly in the testes. It stimulates the development of male secondary sexual characteristics during puberty, including facial and body hair growth, deepening of the voice and increased muscle mass. Testosterone also plays a crucial role in sperm production and maintaining male reproductive tissues.
The menstrual cycle is a complex sequence of hormonal changes that prepares a woman's body for pregnancy each month. It typically lasts about 28 days, though this can vary between individuals. The cycle is controlled by a delicate balance of hormones from the pituitary gland and ovaries.
The menstrual cycle is regulated by four key hormones:
Follicle Stimulating Hormone is released by the pituitary gland. It stimulates the development of follicles in the ovary, each containing an egg. As the follicles grow, they produce oestrogen.
Luteinising Hormone is also released by the pituitary gland. A surge in LH triggers ovulation - the release of a mature egg from the follicle. This usually happens around day 14 of the cycle.
After ovulation, the empty follicle forms the corpus luteum, which produces progesterone and some oestrogen. These hormones prepare the uterus lining for a possible pregnancy.
The menstrual cycle can be divided into three main phases:
If pregnancy doesn't occur, levels of oestrogen and progesterone drop. This causes the thickened uterus lining to break down and be shed through the vagina as a period. Meanwhile, FSH levels begin to rise, starting the development of new follicles in the ovaries.
FSH stimulates follicle development in the ovaries. The growing follicles produce increasing amounts of oestrogen, which causes the lining of the uterus to thicken in preparation for a possible pregnancy. One follicle becomes dominant and continues to mature.
High oestrogen levels trigger a surge of LH from the pituitary gland. This causes the dominant follicle to rupture and release its egg (ovulation). The egg travels down the fallopian tube towards the uterus.
After ovulation, the empty follicle forms the corpus luteum, which produces progesterone and some oestrogen. These hormones maintain the thickened uterus lining. If the egg isn't fertilised, the corpus luteum degenerates, hormone levels fall and the cycle begins again.
The menstrual cycle is controlled by negative and positive feedback mechanisms. For example, as oestrogen levels rise during the follicular phase, they initially inhibit FSH production (negative feedback). However, just before ovulation, high oestrogen levels actually stimulate LH release (positive feedback), triggering ovulation. This demonstrates how hormones can have different effects depending on their concentration and timing.
If an egg is fertilised, a series of hormonal changes ensure the pregnancy is maintained and develops normally.
After fertilisation, the developing embryo produces human chorionic gonadotropin (hCG). This hormone signals the corpus luteum to continue producing progesterone and oestrogen, preventing the breakdown of the uterus lining. Later, the placenta takes over hormone production. Progesterone levels remain high throughout pregnancy to:
When the baby is ready to be born, hormone levels change dramatically:
Our understanding of sex hormones has led to the development of various contraceptive methods and treatments for infertility.
Hormonal contraceptives work by altering the natural hormone balance to prevent pregnancy. The combined pill contains synthetic versions of oestrogen and progesterone that:
The progesterone-only pill (mini-pill) works mainly by thickening cervical mucus and thinning the uterus lining.
For couples struggling to conceive, hormone treatments can help:
During menopause, a woman's ovaries stop producing eggs and hormone levels decline. This can cause symptoms like hot flushes, mood changes and bone thinning. Hormone Replacement Therapy (HRT) provides artificial hormones to relieve these symptoms. This demonstrates how our understanding of sex hormones can improve quality of life beyond reproductive functions.
Sex hormones are essential chemical messengers that coordinate reproductive functions in humans. They control sexual development during puberty, regulate the menstrual cycle, maintain pregnancy and trigger birth. Our understanding of these hormones has led to advances in contraception and fertility treatments, giving people more control over their reproductive health.
The complex interplay between different hormones demonstrates the sophistication of the body's coordination systems. Through negative and positive feedback mechanisms, hormone levels are carefully regulated to ensure reproductive processes occur at the right time and in the right sequence.
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