The female reproductive system is entirely internal. The vagina is the canal that leads from the outside of the body to the cervix, the opening to the uterus.
The uterus is the muscular organ where a fertilised egg, or embryo, attaches and develops. It is the size and shape of a pear and lined with a rich and nourishing membrane, the endometrium.
The fallopian tubes extend from the top of the uterus down over the ovaries, the two walnut-sized organs that contain the eggs.
The eggs in each ovary are produced before a female child is even born. At birth, her ovaries contain about 2 million eggs. Over time, the number of eggs diminishes and none are replaced. By the time a girl has her first period, about 400,000 eggs remain.
Every month from puberty to menopause, eggs begin to mature inside several fluid filled ‘cysts’ within the ovaries, called follicles. Only one of these follicles will become dominant, while the others will shrink and be absorbed by the ovary. At mid-cycle, the dominant follicle releases a single egg during ovulation, which then travels down the fallopian tube toward the uterus.
For a pregnancy to occur, fertilisation happens in the fallopian tube, when the egg meets the sperm. The developing embryo then travels down the fallopian tube to the uterus, where it will implant in the endometrium approximately 7 days after ovulation.
Hormones control the highly complicated sequence of events leading to ovulation. The pituitary gland in the brain produces the two hormones that the ovary needs:
As these hormones are released, the monthly menstrual cycle occurs in three phases:
The follicular phase begins on Day 1 of the menstrual cycle, which is the first day of menstrual bleeding. FSH begins by stimulating the development of many follicles, but as levels of FSH gradually fall in a natural cycle only one follicle will dominate and go on to produce a mature egg. The non-dominant follicles are absorbed by the ovary and cannot be used again. The developing follicle also secretes estrogen, which has several functions. For example, estrogen develops the watery mid-cycle changes in cervical mucus that assist the passage of sperm into the uterus and also causes the thickening of the endometrium required for implantation. The main estrogen the ovary produces is estradiol (E2).
The ovulatory phase is short. It begins when, in response to rising estrogen levels, the level of LH rises dramatically. This LH surge triggers the final maturation of the egg, the rupture of the follicle, and then the release of the egg. This usually happens 14 days before the next period is due, or on Day 14 of a 28-day cycle.
The luteal phase begins after ovulation. At this point, the ovarian follicle where the egg developed, collapses and solidifies to become the corpus luteum. This very important structure mainly produces progesterone, the hormone necessary for transforming the endometrium so that a fertilised egg (the early embryo) can implant and develop.
If conception does not occur, the corpus luteum stops functioning on about Day 26 of a 28-day cycle. Without the support of progesterone, the endometrium begins to break down and is shed in menstruation.
As with women, luteinising hormone (LH) and follicle stimulating hormone (FSH) also control men’s reproductive systems. Unlike women, however, the male reproductive system is both internal and external.
The testes lie in the scrotum, the pouch of skin located beneath the man’s penis. The testes are the organs that produce sperm and testosterone. From the testes, sperm pass slowly through the coiled channels of the epididymis, where they mature.
Once sperm are mature, they move into the vas deferens, a tube that connects the epididymis with the urethra via a common ejaculatory duct. The entire process of sperm formation takes approximately 72 days.
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