The central role of the female reproductive system is carried out by the ovaries, which produce sex cells, called ova or eggs, containing genetic material. When an egg fuses with a male sex cell, called a sperm, it has the potential to develop into a fetus. The ovaries also secrete sex hormones that control sexual development and the menstrual cycle. The breasts play a part in sexual arousal and produce milk after childbirth.
The only visible parts of the female reproductive system are the tissues that make up the vulva. The labia are folds of skin that protect the entrance to the vagina, which is lined with cells that produce a slightly acidic fluid to prevent infection. The vagina leads from the outside of the body to the uterus, the thick-walled organ in which a fetus develops. Two fallopian tubes lead from the uterus to the two ovaries, where eggs are stored.
Newborn girls have a supply of about 150,000 immature eggs, which are present in their ovaries before birth. The eggs are stored in the ovaries and do not begin to mature until a rise in the levels of female sex hormones at puberty triggers the start of monthly menstrual cycles.
Once a month, an egg matures for 14 days in its follicle and is released from the ovary into the fallopian tube in a process called ovulation. The egg survives for 24 hours after ovulation and can only be fertilized by a sperm during this time. It takes about 5–6 days for the egg, fertilized or not, to pass along the tube to the uterus. If fertilized, the egg eventually implants in the lining of the uterus, which becomes thicker after ovulation, ready for an egg to implant. The egg then grows into an embryo. If not fertilized, the egg passes out of the vagina, along with the uterine lining. This blood loss is called menstruation.
The cycle of egg maturation, ovulation, and menstrual bleeding occurs at intervals of, on average, 28 days during a woman’s reproductive life. At the menopause, which usually occurs between the ages of 45 and 55, eggs stop maturing and are no longer released by the ovaries. Menstruation stops, and the reproductive phase of a woman’s life comes to an end.
The ovaries produce the hormones oestrogen and progesterone, but secretion of these is controlled by follicle-stimulating hormone and luteinizing hormone, which are produced in the pituitary gland, a tiny structure just below the brain. Sex hormones control sexual development at puberty, the menstrual cycle, and fertility. Oestrogen also stimulates the fat distribution that results in a woman’s rounded shape. Changes in hormone levels during the menstrual cycle and at menopause may affect mood and behaviour.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.